Numerical Hydrodynamics in General Relativity
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Abstract
The current status of numerical solutions for the equations of ideal general relativistic hydrodynamics is reviewed. With respect to an earlier version of the article, the present update provides additional information on numerical schemes, and extends the discussion of astrophysical simulations in general relativistic hydrodynamics. Different formulations of the equations are presented, with special mention of conservative and hyperbolic formulations welladapted to advanced numerical methods. A large sample of available numerical schemes is discussed, paying particular attention to solution procedures based on schemes exploiting the characteristic structure of the equations through linearized Riemann solvers. A comprehensive summary of astrophysical simulations in strong gravitational fields is presented. These include gravitational collapse, accretion onto black holes, and hydrodynamical evolutions of neutron stars. The material contained in these sections highlights the numerical challenges of various representative simulations. It also follows, to some extent, the chronological development of the field, concerning advances on the formulation of the gravitational field and hydrodynamic equations and the numerical methodology designed to solve them.
Keywords
Black Hole Neutron Star Gravitational Wave Smooth Particle Hydrodynamic Gravitational Collapse1 Introduction
The description of important areas of modern astronomy, such as highenergy astrophysics or gravitational wave astronomy, requires general relativity. Highenergy radiation is often emitted by highly relativistic events in regions of strong gravitational fields near compact objects such as neutron stars or black holes. The production of relativistic radio jets in active galactic nuclei, explained by pure hydrodynamical effects as in the twinexhaust model [35], by hydromagnetic centrifugal acceleration as in the BlandfordPayne mechanism [34], or by electromagnetic extraction of energy as in the BlandfordZnajek mechanism [36], involves an accretion disk around a rotating supermassive black hole. The discovery of kHz quasiperiodic oscillations in lowmass Xray binaries extended the frequency range over which these oscillations occur into timescales associated with the relativistic, innermost regions of accretion disks (see, e.g., [288]). A relativistic description is also necessary in scenarios involving explosive collapse of very massive stars (∼30M_{⊙}) to a black hole (in the socalled collapsar and hypernova models), or during the last phases of the coalescence of neutron star binaries. These catastrophic events are believed to exist at the central engine of highly energetic γray bursts (GRBs) [215, 201, 307, 216]. In addition, nonspherical gravitational collapse leading to black hole formation or to a supernova explosion, and neutron star binary coalescence are among the most promising sources of detectable gravitational radiation. Such astrophysical scenarios constitute one of the main targets for the new generation of groundbased laser interferometers, just starting their gravitational wave search (LIGO, VIRGO, GEO600, TAMA) [286, 206].
A powerful way to improve our understanding of the above scenarios is through accurate, large scale, threedimensional numerical simulations. Nowadays, computational general relativistic astrophysics is an increasingly important field of research. In addition to the large amount of observational data by highenergy X and γray satellites such as Chandra, XMMNewton, or INTEGRAL, and the new generation of gravitational wave detectors, the rapid increase in computing power through parallel supercomputers and the associated advance in software technologies is making possible large scale numerical simulations in the framework of general relativity. However, the computational astrophysicist and the numerical relativist face a daunting task. In the most general case, the equations governing the dynamics of relativistic astrophysical systems are an intricate, coupled system of timedependent partial differential equations, comprising the (general) relativistic (magneto)hydrodynamic (MHD) equations and the Einstein gravitational field equations. In many cases, the number of equations must be augmented to account for nonadiabatic processes, e.g., radiative transfer or sophisticated microphysics (realistic equations of state for nuclear matter, nuclear physics, magnetic fields, and so on).
Nevertheless, in some astrophysical situations of interest, e.g., accretion of matter onto compact objects or oscillations of relativistic stars, the “test fluid” approximation is enough to get an accurate description of the underlying dynamics. In this approximation the fluid selfgravity is neglected in comparison to the background gravitational field. This is best exemplified in accretion problems where the mass of the accreting fluid is usually much smaller than the mass of the compact object. Additionally, a description employing ideal hydrodynamics (i.e., with the stressenergy tensor being that of a perfect fluid), is also a fairly standard choice in numerical astrophysics.
The main purpose of this review is to summarize the existing efforts to solve numerically the equations of (ideal) general relativistic hydrodynamics. To this aim, the most important numerical schemes will be presented first in some detail. Prominence will be given to the socalled Godunovtype schemes written in conservative form. Since [163], it has been demonstrated gradually [93, 78, 244, 83, 21, 297, 229] that conservative methods exploiting the hyperbolic character of the relativistic hydrodynamic equations are optimally suited for accurate numerical integrations, even well inside the ultrarelativistic regime. The explicit knowledge of the characteristic speeds (eigenvalues) of the equations, together with the corresponding eigenvectors, provides the mathematical (and physical) framework for such integrations, by means of either exact or approximate Riemann solvers.
The article includes, furthermore, a comprehensive description of “relevant” numerical applications in relativistic astrophysics, including gravitational collapse, accretion onto compact objects, and hydrodynamical evolution of neutron stars. Numerical simulations of strongfield scenarios employing Newtonian gravity and hydrodynamics, as well as possible postNewtonian extensions, have received considerable attention in the literature and will not be covered in this review, which focuses on relativistic simulations. Nevertheless, we must emphasize that most of what is known about hydrodynamics near compact objects, in particular in black hole astrophysics, has been accurately described using Newtonian models. Probably the best known example is the use of a pseudoNewtonian potential for nonrotating black holes that mimics the existence of an event horizon at the Schwarzschild gravitational radius [217]. This has allowed accurate interpretations of observational phenomena.
The organization of this article is as follows: Section 2 presents the equations of general relativistic hydrodynamics, summarizing the most relevant theoretical formulations that, to some extent, have helped to drive the development of numerical algorithms for their solution. Section 3 is mainly devoted to describing numerical schemes specifically designed to solve nonlinear hyperbolic systems of conservation laws. Hence, particular emphasis will be paid on conservative highresolution shockcapturing (HRSC) upwind methods based on linearized Riemann solvers. Alternative schemes such as smoothed particle hydrodynamics (SPH), (pseudo)spectral methods, and others will be briefly discussed as well. Section 4 summarizes a comprehensive sample of hydrodynamical simulations in strongfield general relativistic astrophysics. Finally, in Section 5 we provide additional technical information needed to build up upwind HRSC schemes for the general relativistic hydrodynamics equations. Geometrized units (G=c=1) are used throughout the paper except where explicitly indicated, as well as the metric conventions of [186]. Greek (Latin) indices run from 0 to 3 (1 to 3).
2 Equations of General Relativistic Hydrodynamics
As usual, \({\nabla _\mu }\) stands for the covariant derivative associated with the fourdimensional spacetime metric \({g_{\mu \nu }}\). The density current is given by \({J_\mu } = \rho {u^\mu },\;{u^\mu }\) representing the fluid 4velocity and ρ the restmass density in a locally inertial reference frame.
Due to their simplicity, the most widely employed EOSs in numerical simulations are the ideal fluid EOS, p=(Γ−1)ρε, where Γ is the adiabatic index, and the polytropic EOS (e.g., to build equilibrium stellar models), \(p = K{\rho ^\Gamma } \equiv K{\rho ^{1 + 1/N}}\), K being the polytropic constant and N the polytropic index.
Traditionally, most of the approaches for numerical integrations of the general relativistic hydrodynamic equations have adopted spacelike foliations of the spacetime, within the 3+1 formulation. Recently, however, covariant forms of these equations, well suited for advanced numerical methods, have also been developed. This is reviewed next in a chronological way.
2.1 Spacelike 3+1 approaches
In the 3+1 (ADM) formulation [15], spacetime is foliated into a set of nonintersecting spacelike hypersurfaces. There are two kinematic variables describing the evolution between these surfaces: the lapse function α, which describes the rate of advance of time along a timelike unit vector \({n^\mu }\) normal to a surface, and the spacelike shift vector β^{ i } that describes the motion of coordinates within a surface.
2.1.1 1+1 Lagrangian formulation (May and White)
The pioneering numerical work in general relativistic hydrodynamics dates back to the onedimensional gravitational collapse code of May and White [172, 173]. Building on theoretical work by Misner and Sharp [185], May and White developed a timedependent numerical code to solve the evolution equations describing adiabatic spherical collapse in general relativity. This code was based on a Lagrangian finite difference scheme (see Section 3.1), in which the coordinates are comoving with the fluid. artificial viscosity terms were included in the equations to damp the spurious numerical oscillations caused by the presence of shock waves in the flow solution. May and White’s formulation became the starting point of a large number of numerical investigations in subsequent years and, hence, it is worth describing its main features in some detail.
Hydrodynamics codes based on the original formulation of May and White and on later versions (e.g., [293]) have been used in many nonlinear simulations of supernova and neutron star collapse (see, e.g., [184, 280] and references therein), as well as in perturbative computations of spherically symmetric gravitational collapse within the framework of the linearized Einstein equations [251, 252]. In Section 4.1.1 below, some of these simulations are discussed in detail. An interesting analysis of the above formulation in the context of gravitational collapse is provided by Miller and Sciama [182]. By comparing the Newtonian and relativistic equations, these authors showed that the net acceleration of the infalling mass shells is larger in general relativity than in Newtonian gravity. The Lagrangian character of May and White’s formulation, together with other theoretical considerations concerning the particular coordinate gauge, has prevented its extension to multidimensional calculations. However, for onedimensional problems, the Lagrangian approach adopted by May and White has considerable advantages with respect to an Eulerian approach with spatially fixed coordinates, most notably the lack of numerical diffusion.
2.1.2 3+1 Eulerian formulation (Wilson)
A direct inspection of the system shows that the equations are written as a coupled set of advection equations. In doing so, the terms containing derivatives (in space or time) of the pressure are treated as source terms. This approach, hence, sidesteps an important guideline for the formulation of nonlinear hyperbolic systems of equations, namely the preservation of their conservation form. This is a necessary condition to guarantee correct evolution in regions of sharp entropy generation (i.e., shocks). Furthermore, some amount of numerical dissipation must be used to stabilize the solution across discontinuities. In this spirit, the first attempt to solve the equations of general relativistic hydrodynamics in the original Wilson’s scheme [300] used a combination of finite difference upwind techniques with artificial viscosity terms. Such terms adapted the classic treatment of shock waves introduced by von Neumann and Richtmyer [295] to the relativistic regime (see Section 3.1.1).
As mentioned before, the description of the evolution of selfgravitating matter fields in general relativity requires a joint integration of the hydrodynamic equations and the gravitational field equations (the Einstein equations). Using Wilson’s formulation for the fluid dynamics, such coupled simulations were first considered in [301], building on a vacuum numerical relativity code specifically developed to investigate the headon collision of two black holes [273]. The resulting code was axially symmetric and aimed to integrate the coupled set of equations in the context of stellar core collapse [82].
Very recently, Anninos and Fragile [13] have compared stateoftheart artificial viscosity schemes and highorder nonoscillatory central schemes (see Section 3.1.3) using Wilson’s formulation for the former class of schemes and a conservative formulation (similar to the one considered in [221, 218]; Section 2.2.2) for the latter. Using a threedimensional Cartesian code, these authors found that earlier results for artificial viscosity schemes in shock tube tests or shock reflection tests are not improved, i.e., the numerical solution becomes increasingly unstable for shock velocities greater than about ∼0.95c. On the other hand, results for the shock reflection problem with a secondorder finite difference central scheme show the suitability of such a scheme to handle ultrarelativistic flows, the underlying reason being, most likely, the use of a conservative formulation of the hydrodynamic equations rather than the particular scheme employed (see Section 3.1.3). Tests concerning spherical accretion onto a Schwarzschild black hole using both schemes yield the maximum relative errors near the event horizon, as large as ∼24% for the central scheme.
2.1.3 3+1 conservative Eulerian formulation (Ibáñez and coworkers)
In 1991, Martí, Ibáñez, and Miralles [163] presented a new formulation of the (Eulerian) general relativistic hydrodynamic equations. This formulation was aimed to take fundamental advantage of the hyperbolic and conservative character of the equations, contrary to the one discussed in the previous section. From the numerical point of view, the hyperbolic and conservative nature of the relativistic Euler equations allows for the use of schemes based on the characteristic fields of the system, bringing to relativistic hydrodynamics the existing tools of classical fluid dynamics. This procedure departs from earlier approaches, most notably in avoiding the need for either artificial dissipation terms to handle discontinuous solutions [300, 301], or implicit schemes as proposed in [208].
If a numerical scheme written in conservation form converges, it automatically guarantees the correct RankineHugoniot (jump) conditions across discontinuities — the shockcapturing property (see, e.g., [152]). Writing the relativistic hydrodynamic equations as a system of conservation laws, identifying the suitable vector of unknowns, and building up an approximate Riemann solver permitted the extension of stateoftheart highresolution shockcapturing schemes (HRSC in the following) from classical fluid dynamics into the realm of relativity [163].
The approach followed in [93] for the equations of special relativistic hydrodynamics was extended to general relativity in [21]. The choice of evolved variables (conserved quantities) in the 3+1 Eulerian formulation developed by Banyuls et al. [21] differs slightly from that of Wilson’s formulation [300]. It comprises the restmass density (D), the momentum density in the jdirection (S_{ j } ), and the total energy density (E), measured by a family of observers which are the natural extension (for a generic spacetime) of the Eulerian observers in classical fluid dynamics. Interested readers are directed to [21] for more complete definitions and geometrical foundations.
The local characteristic structure of the previous system of equations was presented in [21]. The eigenvalues (characteristic speeds) of the corresponding Jacobian matrices are all real (but not distinct, one showing a threefold degeneracy as a result of the assumed directional splitting approach), and a complete set of righteigenvectors exists. System (30) satisfies, hence, the definition of hyperbolicity. As it will become apparent in Section 3.1.2 below, the knowledge of the spectral information is essential in order to construct HRSC schemes based on Riemann solvers. This information can be found in [21] (see also [96]).
The range of applications considered so far in general relativity employing the above formulation of the hydrodynamic equations, Equation (30), (31), (32), (33), is still small and mostly devoted to the study of stellar core collapse and accretion flows onto black holes (see Sections 4.1.1 and 4.2 below). In the special relativistic limit this formulation is being successfully applied to simulate the evolution of (ultra)relativistic extragalactic jets, using numerical models of increasing complexity (see, e.g., [167, 8]). The first applications in general relativity were performed, in one spatial dimension, in [163], using a slightly different form of the equations. Preliminary investigations of gravitational stellar collapse were attempted by coupling the above formulation of the hydrodynamic equations to a hyperbolic formulation of the Einstein equations developed by [39]. These results are discussed in [161, 38]. More recently, successful evolutions of fully dynamical spacetimes in the context of adiabatic stellar core collapse, both in spherical symmetry and in axisymmetry, have been achieved [129, 244, 67]. These investigations are considered in Section 4.1.1 below.
An ambitious threedimensional, Eulerian code which evolves the coupled system of Einstein and hydrodynamics equations was developed by Font et al. [96] (see Section 3.3.2). The formulation of the hydrodynamic equations in this code follows the conservative Eulerian approach discussed in this section. The code is constructed for a completely general spacetime metric based on a Cartesian coordinate system, with arbitrarily specifiable lapse and shift conditions. In [96] the spectral decomposition (eigenvalues and righteigenvectors) of the general relativistic hydrodynamic equations, valid for general spatial metrics, was derived, extending earlier results of [21] for nondiagonal metrics. A complete set of lefteigenvectors was presented by Ibáñez et al. [127]. Due to the paramount importance of the characteristic structure of the equations in the design of upwind HRSC schemes based upon Riemann solvers, we summarize all necessary information in Section 5.2 of this article.
2.2 Covariant approaches
General (covariant) conservative formulations of the general relativistic hydrodynamic equations for ideal fluids, i.e., not restricted to spacelike foliations, have been reported in [78] and, more recently, in [221, 218]. The form invariance of these approaches with respect to the nature of the spacetime foliation implies that existing work on highly specialized techniques for fluid dynamics (i.e., HRSC schemes) can be adopted straightforwardly. In the next two sections we describe the existing covariant formulations in some detail.
2.2.1 Eulderink and Mellema
Eulderink and Mellema [76, 78] first derived a covariant formulation of the general relativistic hydrodynamic equations. As in the formulation discussed in Section 2.1.3, these authors took special care to use the conservative form of the system, with no derivatives of the dependent fluid variables appearing in the source terms. Additionally, this formulation is strongly adapted to a particular numerical method based on a generalization of Roe’s approximate Riemann solver. Such a solver was first applied to the nonrelativistic Euler equations in [242] and has been widely employed since in simulating compressible flows in computational fluid dynamics. Furthermore, their procedure is specialized for a perfect fluid EOS, p=(Γ1)ρε, Γ being the (constant) adiabatic index of the fluid.
Eulderink and Mellema computed the exact “Roe matrix” [242] for the vector (34) and obtained the corresponding spectral decomposition. The characteristic information is used to solve the system numerically using Roe’s generalized approximate Riemann solver. Roe’s linearization can be expressed in terms of the average state \(\tilde \omega = \left( {{\omega _{\rm{L}}} + {\omega _{\rm{R}}}} \right)/\left( {{K_{\rm{L}}} + {K_{\rm{R}}}} \right)\), where L and R denote the left and right states in a Riemann problem (see Section 3.1.2). Further technical details can be found in [76, 78].
The performance of this general relativistic Roe solver was tested in a number of onedimensional problems for which exact solutions are known, including nonrelativistic shock tubes, special relativistic shock tubes, and spherical accretion of dust and a perfect fluid onto a (static) Schwarzschild black hole. In its special relativistic version it has been used in the study of the confinement properties of relativistic jets [77]. However, no astrophysical applications in strongfield general relativistic flows have yet been attempted with this formulation.
2.2.2 Papadopoulos and Font
The state of the fluid is uniquely described using either vector of variables, i.e., either U or w, and each one can be obtained from the other via the definitions (35)) and the use of the normalization condition for the 4velocity, \({g_{\mu \nu }}{u^\mu }{u^\nu } =  1\). The local characteristic structure of the above system of equations was presented in [221], where the formulation proved well suited for the numerical implementation of HRSC schemes. The formulation presented in this section has been developed for a perfect fluid EOS. Extensions to account for generic EOS are given in [218]. This reference further contains a comprehensive analysis of general relativistic hydrodynamics in conservation form.

Lightcone hydrodynamics: A comprehensive numerical study of the formulation of the hydrodynamic equations discussed in this section was presented in [221], where it was applied to simulate onedimensional relativistic flows on null (lightlike) spacetime foliations. The various demonstrations performed include standard shock tube tests in Minkowski spacetime, perfect fluid accretion onto a Schwarzschild black hole using ingoing null EddingtonFinkelstein coordinates, dynamical spacetime evolutions of relativistic polytropes (i.e., stellar models satisfying the socalled TolmanOppenheimerVolkoff equations of hydrostatic equilibrium) sliced along the radial null cones, and accretion of selfgravitating matter onto a central black hole.
Procedures for integrating various forms of the hydrodynamic equations on null hypersurfaces are much less common than on spacelike (3+1) hypersurfaces. They have been presented before in [133] (see [31] for a recent implementation). This approach is geared towards smooth isentropic flows. A Lagrangian method, limited to spherical symmetry, was developed by [181]. Recent work in [71] includes an Eulerian, nonconservative, formulation for general fluids in null hypersurfaces and spherical symmetry, including their matching to a spacelike section.
The general formalism laid out in [221, 218] is currently being systematically applied to astrophysical problems of increasing complexity. Applications in spherical symmetry include the investigation of accreting dynamic black holes, which can be found in [221, 222]. Studies of the gravitational collapse of supermassive stars are discussed in [156], and studies of the interaction of scalar fields with relativistic stars are presented in [270]. Axisymmetric neutron star spacetimes have been considered in [269], as part of a broader program aimed at the study of relativistic stellar dynamics and gravitational collapse using characteristic numerical relativity. We note that there has been already a proofofprinciple demonstration of the inclusion of matter fields in three dimensions [31].
2.3 Going further: Nonideal hydrodynamics
Formulations of the equations of nonideal hydrodynamics in general relativity are also available in the literature. The term “nonideal” accounts for additional physics such as viscosity, magnetic fields, and radiation. These nonadiabatic effects can play a major role in some astrophysical systems as, such as accretion disks or relativistic stars.
The equations of viscous hydrodynamics, the NavierStokesFourier equations, have been formulated in relativity in terms of causal dissipative relativistic fluids (see the Living Reviews article by Müller [192] and references therein). These extended fluid theories, however, remain unexplored, numerically, in astrophysical systems. The reason may be the lack of an appropriate formulation wellsuited for numerical studies. Work in this direction was done by Peitz and Appl [224] who provided a 3+1 coordinatefree representation of different types of dissipative relativistic fluid theories which possess, in principle, the potentiality of being well adapted to numerical applications.
The inclusion of magnetic fields and the development of formulations for the MHD equations, attractive to numerical studies, is still very limited in general relativity. Numerical approaches in special relativity are presented in [143, 291, 20]. In particular, Komissarov [143], and Balsara [20] developed two different upwind HRSC (or Godunovtype) schemes, providing the characteristic information of the corresponding system of equations, and proposed a battery of tests to validate numerical MHD codes. 3+1 representations of relativistic MHD can be found in [272, 80]. In [313] the transport of energy and angular momentum in magnetohydrodynamical accretion onto a rotating black hole was studied adopting Wilson’s formulation for the hydrodynamic equations (conveniently modified to account for the magnetic terms), and the magnetic induction equation was solved using the constrained transport method of [80]. Recently, Koide et al. [141, 142] performed the first MHD simulation, in general relativity, of magnetically driven relativistic jets from an accretion disk around a Schwarzschild black hole (see Section 4.2.2). These authors used a secondorder finite difference central scheme with nonlinear dissipation developed by Davis [61]. Even though astrophysical applications of Godunovtype schemes (see Section 3.1.2) in general relativistic MHD are still absent, it is realistic to believe this situation may change in the near future.
The interaction between matter and radiation fields, present in different levels of complexity in all astrophysical systems, is described by the equations of radiation hydrodynamics. The Newtonian framework is highly developed (see, e.g., [180]; the special relativistic transfer equation is also considered in that reference). Pons et al. [230] discuss a hyperbolic formulation of the radiative transfer equations, paying particular attention to the closure relations and to extend HRSC schemes to those equations. General relativistic formulations of radiative transfer in curved spacetimes are considered in, e.g., [237] and [316] (see also references therein).
3 Numerical Schemes
We turn now to describe the numerical schemes, mainly those based on finite differences, specifically designed to solve nonlinear hyperbolic systems of conservation laws. As discussed in the previous section, the equations of general relativistic hydrodynamics fall in this category, irrespective of the formulation. Even though we also consider schemes based on artificial viscosity techniques, the emphasis is on the socalled highresolution shockcapturing (HRSC) schemes (or Godunovtype methods), based on (either exact or approximate) solutions of local Riemann problems using the characteristic structure of the equations. Such finite difference schemes (or, in general, finite volume schemes) have been the subject of diverse review articles and textbooks (see, e.g., [152, 153, 287, 128]). For this reason only the most relevant features will be covered here, addressing the reader to the appropriate literature for further details. In particular, an excellent introduction to the implementation of HRSC schemes in special relativistic hydrodynamics is presented in the Living Reviews article by Martí and Müller [164]. Alternative techniques to finite differences, such as smoothed particle hydrodynamics, (pseudo)spectral methods and others, are briefly considered last.
3.1 Finite difference schemes
Any system of equations presented in the previous section can be solved numerically by replacing the partial derivatives by finite differences on a discrete numerical grid, and then advancing the solution in time via some timemarching algorithm. Hence, specification of the state vector U on an initial hypersurface, together with a suitable choice of EOS, followed by a recovery of the primitive variables, leads to the computation of the fluxes and source terms. Through this procedure the first time derivative of the data is obtained, which then leads to the formal propagation of the solution forward in time, with a time step constrained by the CourantFriedrichsLewy (CFL) condition.
The hydrodynamic equations (either in Newtonian physics or in general relativity) constitute a nonlinear hyperbolic system and, hence, smooth initial data can transform into discontinuous data (the crossing of characteristics in the case of shocks) in a finite time during the evolution. As a consequence, classical finite difference schemes (see, e.g., [152, 287]) present important deficiencies when dealing with such systems. Typically, firstorder accurate schemes are much too dissipative across discontinuities (excessive smearing) and second order (or higher) schemes produce spurious oscillations near discontinuities, which do not disappear as the grid is refined. To avoid these effects, standard finite difference schemes have been conveniently modified in various ways to ensure highorder, oscillationfree accurate representations of discontinuous solutions, as we discuss next.
3.1.1 Artificial viscosity approach
The main advantage of the artificial viscosity approach is its simplicity, which results in high computational efficiency. Experience has shown, however, that this procedure is both problem dependent and inaccurate for ultrarelativistic flows [208, 13]. Furthermore, the artificial viscosity approach has the inherent ambiguity of finding the appropriate form for Q that introduces the necessary amount of dissipation to reduce the spurious oscillations and, at the same time, avoids introducing excessive smearing in the discontinuities. In many instances both properties are difficult to achieve simultaneously. A comprehensive numerical study of artificialviscosityinduced errors in strong shock calculations in Newtonian hydrodynamics (including also proposed improvements) was presented by Noh [207].
3.1.2 Highresolution shockcapturing (HRSC) upwind schemes
In finite difference schemes, convergence properties under grid refinement must be enforced to ensure that the numerical results are correct (i.e., if a scheme with an order of accuracy α is used, the global error of the numerical solution has to tend to zero as \(\mathcal{O}{(\Delta x)^\alpha }\) as the cell width Δx tends to zero). For hyperbolic systems of conservation laws, schemes written in conservation form are preferred since, according to the LaxWendroff theorem [150], they guarantee that the convergence, if it exists, is to one of the socalled weak solutions of the original system of equations. Such weak solutions are generalized solutions that satisfy the integral form of the conservation system. They are C^{s1} classical solutions (continuous and differentiable) in regions where they are continuous and have a finite number of discontinuities.
A numerical scheme is said to be TVstable if TV(u^{ n }) is bounded for all Δt at any time for each initial data. In the case of nonlinear, scalar conservation laws it can be proved that TVstability is a sufficient condition for convergence [152], as long as the numerical schemes are written in conservation form and have consistent numerical flux functions. Current research has focused on the development of highresolution numerical schemes in conservation form satisfying the condition of TVstability, such as the socalled total variation diminishing (TVD) schemes [115] (see below).
A numerical scheme written in conservation form ensures that, in the absence of sources, the (physically) conserved quantities, according to the partial differential equations, are numerically conserved by the finite difference equations.
For reasons of numerical efficiency and, particularly in multidimensions, the exact solution of the Riemann problem is frequently avoided and linearized (approximate) Riemann solvers are preferred. These solvers are based on the exact solution of Riemann problems corresponding to a linearized version of the original system of equations. After extensive experimentation, the results achieved with approximate Riemann solvers are comparable to those obtained with the exact solver (see [287] for a comprehensive overview of Godunovtype methods, and [164] for an excellent summary of approximate Riemann solvers in special relativistic hydrodynamics).
The way these variables are computed determines the spatial order of accuracy of the numerical algorithm and controls the amplitude of the local jumps at every cell interface. If these jumps are monotonically reduced, the scheme provides more accurate initial guesses for the solution of the local Riemann problems (the average values give only firstorder accuracy). A wide variety of cell reconstruction procedures is available in the literature. Among the slope limiter procedures (see, e.g., [287, 153]) most commonly used for TVD schemes [115] are the second order, piecewiselinear reconstruction, introduced by van Leer [290] in the design of the MUSCL scheme (Monotonic Upstream Scheme for Conservation Laws), and the third order, piecewise parabolic reconstruction developed by Colella and Woodward [58] in their Piecewise Parabolic Method (PPM). Since TVD schemes are only firstorder accurate at local extrema, alternative reconstruction procedures for which some growth of the total variation is allowed have also been developed. Among those, we mention the total variation bounded (TVB) schemes [268] and the essentially nonoscillatory (ENO) schemes [116].
Alternatively, highorder methods for nonlinear hyperbolic systems have also been designed using flux limiters rather than slope limiters, as in the FCT scheme of Boris and Book [46]. In this approach, the numerical flux consists of two pieces, a highorder flux (e.g., the LaxWendroff flux) for smooth regions of the flow, and a loworder flux (e.g., the flux from some monotone method) near discontinuities, \(\mathbf{\hat F} = {\mathbf{\hat F}_{\rm{h}}}  (1  \Phi )({\mathbf{\hat F}_{\rm{h}}}  {\mathbf{\hat F}_{\rm{l}}})\) with the limiter Ф ∈ [0, 1] (see [287, 153] for further details).
During the last few years most of the standard Riemann solvers developed in Newtonian fluid dynamics have been extended to relativistic hydrodynamics: Eulderink [78], as discussed in Section 2.2.1, explicitly derived a relativistic Roe Riemann solver [242]; Schneider et al. [250] carried out the extension of Harten, Lax, van Leer, and Einfeldt’s (HLLE) method [117, 75]; Martí and Müller [166] extended the PPM method of Woodward and Colella [306]; Wen et al. [297] extended Glimm’s exact Riemann solver; Dolezal and Wong [68] put into practice ShuOsher ENO techniques; Balsara [19] extended Colella’s twoshock approximation, and Donat et al. [69] extended Marquina’s method [70]. Recently, much effort has been spent concerning the exact special relativistic Riemann solver and its extension to multidimensions [165, 231, 238, 239]. The interested reader is addressed to [164] and references therein for a comprehensive description of such solvers in special relativistic hydrodynamics.
3.1.3 Highorder central schemes
The use of highorder nonoscillatory symmetric (central) TVD schemes for solving hyperbolic systems of conservation laws emerged at the mid 1980s [61, 243, 311, 205] (see also [312] and [287] and references therein) as an alternative approach to upwind HRSC schemes. Symmetric schemes are based on either highorder schemes (e.g., LaxWendroff) with additional dissipative terms [61, 243, 311], or on nonoscillatory highorder extensions of firstorder central schemes (e.g., LaxFriedrichs) [205]. One of the nicest properties of central schemes is that they exploit the conservation form of the LaxWendroff or LaxFriedrichs schemes. Therefore, they yield the correct propagation speeds of all nonlinear waves appearing in the solution. Furthermore, central schemes sidestep the use of Riemann solvers, which results in enhanced computational efficiency, especially in multidimensional problems. Its use is, thus, not limited to those systems of equations where the characteristic information is explicitly known or to systems where the solution of the Riemann problem is prohibitively expensive to compute. This approach has gradually developed during the last decade to reach a mature status where a number of straightforward central schemes of high order can be applied to any nonlinear hyperbolic system of conservation laws. The typical results obtained for the Euler equations show a quality comparable to that of upwind HRSC schemes, at the expense of a small loss of sharpness of the solution at discontinuities [287]. An uptodate summary of the status and applications of this approach is discussed in [287, 145, 281].
In the context of special and general relativistic MHD, Koide et al. [141, 142] applied a secondorder central scheme with nonlinear dissipation developed by Davis [61] to the study of black hole accretion and formation of relativistic jets. Onedimensional test simulations in special relativistic hydrodynamics performed by the author and coworkers [92] using the SLIC (slope limiter centred) scheme (see [287] for details) showed its capabilities to yield satisfactory results, comparable to those obtained by HRSC schemes based on Riemann solvers, even well inside the ultrarelativistic regime. The slopes of the original central scheme were limited using highorder reconstruction procedures such as PPM [58], which was essential to keep the inherent diffusion of central schemes at discontinuities at reasonable levels. Very recently, Del Zanna and Bucciantini [63] assessed a thirdorder convex essentially nonoscillatory central scheme in multidimensional special relativistic hydrodynamics. Again, these authors obtained results as accurate as those of upwind HRSC schemes in standard tests (shock tubes, shock reflection test). Yet another central scheme has been assessed by [13] in onedimensional special and general relativistic hydrodynamics, where similar results to those of [63] are reported. These authors also validate their central scheme in simulations of spherical accretion onto a Schwarzschild black hole, and further provide a comparison with an artificial viscosity based scheme.
It is worth emphasizing that early pioneer approaches in the field of relativistic hydrodynamics [208, 54] used standard finite difference schemes with artificial viscosity terms to stabilize the solution at discontinuities. However, as discussed in Section 2.1.2, those approaches only succeeded in obtaining accurate results for moderate values of the Lorentz factor (W∼2). A key feature lacking in those investigations was the use of a conservative approach for both the system of equations and the numerical schemes. A posteriori, and in the light of the results reported by [63, 13, 92], it appears that this was the ultimate reason preventing the extension of the early computations to the ultrarelativistic regime.
The alternative of using highorder central schemes instead of upwind HRSC schemes becomes apparent when the spectral decomposition of the hyperbolic system under study is not known. The straightforwardness of a central scheme makes its use very appealing, especially in multidimensions where computational efficiency is an issue. Perhaps the most important example in relativistic astrophysics is the system of (general) relativistic MHD equations. Despite some progress in recent years (see, e.g., [20, 143]), much more work is needed concerning their solution with HRSC schemes based upon Riemann solvers. Meanwhile, an obvious choice is the use of central schemes [141, 142].
3.1.4 Source terms
Most “conservation laws” involve source terms on the right hand side of the equations. In hydrodynamics, for instance, those terms arise when considering external forces such as gravity, which make the right hand side of the momentum and energy equations no longer zero (see Section 2). Other effects leading to the appearance of source terms are radiative heat transfer (accounted for in the energy equation) or ionization (resulting in a collection of nonhomogeneous continuity equations for the mass of each species, which is not conserved separately). The incorporation of the source terms in the solution procedure is a common issue to all numerical schemes considered so far. Since a detailed discussion on the numerical treatment of source terms is beyond the scope of this article, we simply provide some basic information in this section, addressing the interested reader to [287, 153] and references therein for further details.
Correspondingly, the second approach is based on fractional step or splitting methods. The basic idea is to split the equation into different pieces (transport + sources) and apply appropriate methods for each piece independently. In the firstorder Godunov splitting, \({\mathbf{U}^{n + 1}} = {\mathcal L}_{\rm{s}}^{\Delta t}{\mathcal L}_{\rm{f}}^{\Delta t}{\mathbf{U}^n}\), the operator \({\mathcal L}_{\rm{f}}^{\Delta t}\) solves the homogeneous PDE in the first step to yield the intermediate value U*. Then, in the second step, the operator \({\mathcal L}_{\rm{s}}^{\Delta t}\) solves the ordinary differential equation ∂_{ t }U*=S(U*) to yield the final value U^{n+1}. In order to achieve secondorder accuracy (assuming each independent method is second order), a common fractional step method is the Strang splitting, where \({{\bf{U}}^{n + 1}} = {\mathcal L}_{\rm{s}}^{\Delta t/2}{\mathcal L}_{\rm{f}}^{\Delta t}{\mathcal L}_{\rm{s}}^{\Delta t/2}{{\bf{U}}^n}\). Therefore, this method advances by a half time step the solution for the ODE containing the source terms, and by a full time step the conservation law (the PDE) in between each source step.
We note that in some cases the source terms may become stiff, as in phenomena that either occur on much faster timescales than the hydrodynamic time step Δt, or act over much smaller spatial scales than the grid resolution Δx. Stiff source terms may easily lead to numerical difficulties. The interested reader is directed to [153] (and references therein) for further information on various approaches to overcome the problems of stiff source terms.
3.2 Other techniques
Two of the most frequently employed alternatives to finite difference schemes in numerical hydrodynamics are smoothed particle hydrodynamics (SPH) and, to a lesser extent, (pseudo)spectral methods. This section contains a brief description of both approaches. In addition, we also mention the fielddependent variation method and the finite element method. We note, however, that both of these approaches have barely been used so far in relativistic hydrodynamics.
3.2.1 Smoothed particle hydrodynamics
The Lagrangian particle method SPH, derived independently by Lucy [158] and Gingold and Monaghan [104], has shown successful performance to model fluid flows in astrophysics and cosmology. Most studies to date consider Newtonian flows and gravity, enhanced with the inclusion of the fluid selfgravity.
In the SPH method a finite set of extended Lagrangian particles replaces the continuum of hydrodynamical variables, the finite extent of the particles being determined by a smoothing function (the kernel) containing a characteristic length scale h. The main advantage of this method is that it does not require a computational grid, avoiding mesh tangling and distortion. Hence, compared to gridbased finite volume methods, SPH avoids wasting computational power in multidimensional applications, when, e.g., modelling regions containing large voids. Experience in Newtonian hydrodynamics shows that SPH produces very accurate results with a small number of particles (≈10^{3} or even less). However, if more than 10^{4} particles have to be used for certain problems, and selfgravity has to be included, the computational power, which grows as the square of the number of particles, may exceed the capabilities of current supercomputers. Among the limitations of SPH we note the difficulties in modelling systems with extremely different characteristic lengths and the fact that boundary conditions usually require a more involved treatment than in finite volume schemes.
Reviews of the classical SPH equations are abundant in the literature (see, e.g., [187, 191] and references therein). The reader is addressed to [191] for a summary of comparisons between SPH and HRSC methods.
Recently, implementations of SPH to handle (special) relativistic (and even ultrarelativistic) flows have been developed (see, e.g., [57] and references therein). However, SPH has been scarcely applied to simulate relativistic flows in curved spacetimes. Relevant references include [137, 146, 147, 271].
Recently, Siegler and Riffert [271] have developed a Lagrangian conservation form of the general relativistic hydrodynamic equations for perfect fluids (with artificial viscosity) in arbitrary background spacetimes. Within that formulation these authors [271] have built a general relativistic SPH code using the standard SPH formalism as known from Newtonian fluid dynamics (in contrast to previous approaches, e.g., [160, 137, 146]). The conservative character of their scheme has allowed the modelling of ultrarelativistic flows including shocks with Lorentz factors as large as 1000.
3.2.2 Spectral methods
The basic principle underlying spectral methods consists of transforming the partial differential equations into a system of ordinary differential equations by means of expanding the solution in a series on a complete basis. The mathematical theory of these schemes is presented in [110, 52]. Spectral methods are particularly well suited to the solution of elliptic and parabolic equations. Good results can also be obtained for hyperbolic equations as long as no discontinuities appear in the solution. When a discontinuity is present, some amount of artificial viscosity must be added to avoid spurious oscillations. In the specific case of relativistic problems, where coupled systems of elliptic equations (i.e., the Einstein constraint equations) and hyperbolic equations (i.e., hydrodynamics) must be solved, an interesting strategy is to use spectral methods to solve the elliptic equations and HRSC schemes for the hyperbolic ones. Using such combined methods, first results have been obtained in onedimensional supernova collapse simulations, both in the framework of a tensorscalar theory of gravitation [209, 211] and in general relativity [210].
In the nonlinear case, λ≠0, spectral methods proceed in a more convoluted way: First, the derivative of u is computed in the Fourier space. Then, a step back to the configuration space is taken through an inverse Fourier transform. Finally, after multiplying ∂u/∂x by u in the configuration space, the scheme returns again to the Fourier space.
The particular set of trigonometric functions used for the expansion in Equation (60) is chosen because it automatically fulfills the boundary conditions, and because a fast transform algorithm is available (the latter is no longer an issue for today’s computers). If the initial or boundary conditions are not periodic, Fourier expansion is no longer useful because of the presence of a Gibbs phenomenon at the boundaries of the interval. Legendre or Chebyshev polynomials are, in this case, the most common base of functions used in the expansions (see [110, 52] for a discussion on the different expansions).
Extensive numerical applications using (pseudo)spectral methods have been undertaken by the LUTH Relativity Group at the Observatoire de Paris in Meudon. The group has focused on the study of compact objects, as well as the associated violent phenomena of gravitational collapse and supernova explosion. They have developed a fully objectoriented library (based on the C++ computer language) called LORENE [157] to implement (multidomain) spectral methods within spherical coordinates. A comprehensive summary of applications in general relativistic astrophysics is presented in [41]. The most recent ones deal with the computation of quasiequilibrium configurations of either synchronized or irrotational binary neutron stars in general relativity [112, 284, 283]. Such initial data are currently being used by fully relativistic, finite difference timedependent codes (see Section 3.3.2) to simulate the coalescence of binary neutron stars.
3.2.3 Flow fielddependent variation method
Richardson and Chung [240] have recently proposed the flow fielddependent variation (FDV) method as a new approach for general relativistic (nonideal) hydrodynamics computations. In the FDV method, parameters characteristic of the flow field are computed in order to guide the numerical scheme toward a solution. The basic idea is to expand the conservation flow variables into a Taylor series in terms of the FDV parameters, which are related to variations of physical parameters such as the Lorentz factor, the relativistic Reynolds number and the relativistic Froude number.
The physical interpretation of the coefficientts s_{ a } and s_{ b } is the foundation of the FDV method. The firstorder parameter s_{ a } is proportional to \({\mathbf{a}_i}\partial {\mathbf{U}^{n + 1}}/\partial {x_i}\), where a_{ i } is the convection Jacobian representing the change of convective motion. If the Lorentz factor remains constant in space and time, then s_{ a }=0. However, if the Lorentz factor between adjacent zones is large, s_{ a }=1. Similar assessments in terms of the Reynolds number can be provided for the diffusion and diffusion gradients, while the Froude number is used for the source term Jacobian ∂S/∂U. Correspondingly, the secondorder FDV parameters s_{ b } are chosen to be exponentially proportional to the firstorder ones.
Obviously, the main drawback of the FDV method is the dependence of the solution procedure on a large number of problemdependent parameters, a drawback shared to some extent by artificial viscosity schemes. Richardson and Chung [240] have implemented the FDV method in a C++ code called GRAFSS (General Relativistic Astrophysical Flow and Shock Solver). The test problems they report are the special relativistic shock tube (problem 1 in the notation of [166]) and the Bondi accretion onto a Schwarzschild black hole. While their method yields the correct wave propagation, the numerical solution near discontinuities has considerably more diffusion than with upwind HRSC schemes. Nevertheless, the generality of the approach is worth considering. Applications to nonideal hydrodynamics and relativistic MHD are in progress [240].
3.2.4 Going further
The finite element method is the preferred choice to solve multidimensional structural engineering problems since the late 1960s [318]. First steps in the development of the finite element method for modeling astrophysical flows in general relativity are discussed by Meier [176]. The method, designed in a fully covariant manner, is valid not only for the hydrodynamic equations but also for the Einstein and electromagnetic field equations. The most unique aspect of the approach presented in [176] is that the grid can be arbitrarily extended into the time dimension. Therefore, the standard finite element method generalizes to a fourdimensional covariant technique on a spacetime grid, with the engineer’s “isoparametric transformation” becoming the generalized Lorentz transform. At present, the method has shown its suitability to accurately compute the equilibrium stellar structure of Newtonian polytropes, either spherical or rotating. The main limitation of the finite element method, as Meier points out [176], is that it is generally fully implicit, which results in severe computer time and memory limitations for three and fourdimensional problems.
3.3 Stateoftheart threedimensional codes
The most advanced timedependent, finite difference, threedimensional Cartesian codes to solve the system of Einstein and hydrodynamics equations are those developed by Shibata [258] and by the Washington University/NCSA/AEIGolm Numerical Relativity collaboration [96, 89]. The main difference between both codes lies in the numerical methods used to solve the relativistic hydrodynamic equations: artificial viscosity in Shibata’s code [258], and upwind HRSC schemes in the code of [96, 89]. We note, however, that very recently Shibata has upgraded his code to incorporate HRSC schemes (in particular, a Roetype approximate Riemann solver and piecewise parabolic cellreconstruction procedures) [260]. Further 3D codes are currently being developed by a group in the U.S. (Duez et al. [73]) and by a E.U. Research Training Network collaboration [119].
3.3.1 Shibata
In Shibata’s code [258], the hydrodynamics equations are formulated following Wilson’s approach (Section 2.1.2) for a conformaltraceless reformulation of the spacetime variables (see below). An important difference with respect to the original system, Equations ((20), (21), (22)), is that an equation for the entropy is solved instead of the energy equation. The hydrodynamic equations are integrated using van Leer’s [290] second order finite difference scheme with artificial viscosity, following the approach of a precursor code developed by [214].
The ADM Einstein equations are reformulated into a conformal traceless system, an idea originally introduced by Shibata and Nakamura [264] (see also [197]), and further developed by Baumgarte and Shapiro [25]. This “BSSN” formulation, which shows enhanced longterm stability compared to the original ADM system, makes use of a conformal decomposition of the 3metric, \({\tilde \gamma _{ij}} = {e^{  4\phi }}{\gamma _{ij}}\) and the tracefree part of the extrinsic curvature, \({A_{ij}} = {K_{ij}}  {\gamma _{ij}}K/3\), with the conformal factor φ chosen to satisfy \({e^{4\phi }} = {\gamma ^{1/3}} \equiv \det {\left( {{\gamma _{ij}}} \right)^{1/3}}\). In this formulation, as shown by [25], in addition to the evolution equations for the conformal 3metric γ̃_{ ij } and the conformaltraceless extrinsic curvature variables Ã_{ ij }, there are evolution equations for the conformal factor φ, the trace of the extrinsic curvature K and the “conformal connection functions” Γ̃^{ i }. Further details can be found in [25, 258].
The code uses an approximate maximal slicing condition, which amounts to solving a parabolic equation for ln α, and a minimal distortion gauge condition for the shift vector. It also admits πrotation symmetry around the zaxis, as well as plane symmetry with respect to the z=0 plane, allowing computations in a quadrant region. In addition, Shibata has also implemented in the code the “cartoon” method proposed by the AEI Numerical Relativity group [6], which makes possible axisymmetric computations with a Cartesian grid. “Approximate” outgoing boundary conditions are used at the outer boundaries; these do not completely eliminate numerical errors due to spurious back reflection of gravitational waves ^{1}. A staggered leapfrog method is used for the time evolution of the metric quantities. Correspondingly, the hydrodynamic equations are updated using a secondorder twostep RungeKutta scheme. In each time step, the staggered metric quantities needed for the hydrodynamics update are properly extrapolated to intermediate time levels to reach the desired order of accuracy.
The code developed by Shibata [258, 260] has been tested in a variety of problems, including spherical collapse of dust to a black hole, signalled by the appearance of the apparent horizon (comparing 1D and 3D simulations), stability of spherical stars and computation of the radial oscillation period, quadrupole oscillations of perturbed spherical stars and computation of the associated gravitational radiation, preservation of the rotational profile of (approximate) rapidly rotating stars, and the preservation of a corotating binary neutron star in a quasiequilibrium state (assuming a conformally flat 3metric) for more than one orbital period. Improvements of the hydrodynamical schemes have been considered very recently [260], in order to tackle problems in which shocks play an important role, e.g., stellar core collapse to a neutron star. Shibata’s code has allowed important breakthroughs in the study of the morphology and dynamics of various general relativistic astrophysical problems, such as black hole formation resulting from both the gravitational collapse of rotating neutron stars and rotating supermassive stars, and, perhaps most importantly, the coalescence of binary neutron stars, a longstanding problem in numerical relativistic hydrodynamics. These applications are discussed in Section 4. The most recent simulations of binary neutron star coalescence [267] have been performed on a FACOM VPP5000 computer with typical grid sizes of (505, 505, 253) in (x, y, z).
3.3.2 CACTUS/GR ASTRO
A threedimensional general relativistic hydrodynamics code developed by the Washington University/NCSA/AEIGolm collaboration for the NASA Neutron Star Grand Challenge Project [296] is discussed in Refs. [96, 89]. The code is built upon the Cactus Computational Toolkit [171]. A version of this code that passed the milestone requirement of the NASA Grand Challenge project was released to the community. This code has been benchmarked at over 140 GFlop/sec on a 1024 node Cray T3E, with a scaling efficiency of over 95%, showing the potential for large scale 3D simulations of realistic astrophysical systems.
The hydrodynamics part of the code uses the conservative formulation discussed in Section 2.1.3. A variety of stateoftheart Riemann solvers are implemented, including a Roelike solver [242] and Marquina’s flux formula [70]. The code uses slopelimiter methods to construct secondorder TVD schemes by means of monotonic piecewise linear reconstructions of the cellcentered quantities for the computation of the numerical fluxes. The standard “minmod” limiter and the “monotonized centraldifference” limiter [289] are implemented. Both schemes provide the desired secondorder accuracy for smooth solutions, while still satisfying the TVD property. In addition, thirdorder piecewise parabolic (PPM) reconstruction has been recently implemented and used in [279].
The Einstein equations are solved using the following different approaches: (i) the standard ADM formalism, (ii) a hyperbolic formulation developed by [40], and (iii) the BSSN formulation of [197, 264, 25]. The time evolution of both the ADM and the BSSN systems can be performed using several numerical schemes [96, 4, 89]. Currently, a leapfrog (nonstaggered in time), and an iterative CrankNicholson scheme have been coupled to the hydrodynamics solver. The code is designed to handle arbitrary shift and lapse conditions, which can be chosen as appropriate for a given spacetime simulation. The AEI Numerical Relativity group [170] is currently developing robust gauge conditions for (vacuum) black hole spacetimes (see, e.g., [7] and references therein). Hence, all advances accomplished here can be incorporated in future versions of the code for nonvacuum spacetime evolutions. Similarly, since it is a general purpose code, a number of different boundary conditions can be imposed for either the spacetime variables or for the hydrodynamical variables. We refer the reader to [96, 4, 89] for additional details.
The code has been subjected to a series of convergence tests [96], with many different combinations of the spacetime and hydrodynamics finite differencing schemes, demonstrating the consistency of the discrete equations with the differential equations. The simulations performed in [96] include, among others, the evolution of equilibrium configurations of compact stars (solutions to the TOV equations) and the evolution of relativistically boosted TOV stars (v=0.87c) transversing diagonally across the computational domain — a test for which an exact solution exists. In [4, 5] the improved stability properties of the BSSN formulation of the Einstein equations were compared to the ADM system. In particular, in [4] a number of strongly gravitating systems were simulated, including weak and strong gravitational waves, black holes, boson stars and relativistic stars. while the error growthrate can be decreased by going to higher grid resolutions, the BSSN formulation requires grid resolutions higher than the ones needed in the ADM formulation to achieve the same accuracy. Furthermore, it was shown in [89] that the code successfully passed stringent longterm evolution tests, such as the evolution of both spherical and rapidly rotating, stationary stellar configurations, and the formation of an apparent horizon during the collapse of a relativistic star to a black hole. The high accuracy of the hydrodynamical schemes employed has allowed the detailed investigation of the frequencies of radial, quasiradial and quadrupolar oscillations of relativistic stellar models, and use them as a strong assessment of the accuracy of the code. The frequencies obtained have been compared to the frequencies computed with perturbative methods and in axisymmetric nonlinear evolutions [88]. In all of the cases considered, the code reproduces these results with excellent accuracy and is able to extract the gravitational waveforms that might be produced during nonradial stellar pulsations.
4 Hydrodynamical Simulations in Relativistic Astrophysics
With the exception of the vacuum twobody problem (i.e., the coalescence of two black holes), all realistic astrophysical systems and sources of gravitational radiation involve matter. Not surprisingly, the joint integration of the equations of motion for matter and geometry was in the minds of theorists from the very beginning of numerical relativity.
Nowadays there is a large body of numerical investigations in the literature dealing with hydrodynamical integrations in static background spacetimes. Most of those are based on Wilson’s Eulerian formulation of the hydrodynamic equations and use schemes based on finite differences with some amount of artificial viscosity. The use of conservative formulations of the equations, and the incorporation of the characteristic information in the design of numerical schemes has begun in more recent years.
On the other hand, timedependent simulations of selfgravitating flows in general relativity (evolving the spacetime dynamically with the Einstein equations coupled to a hydrodynamic source) constitute a much smaller sample. Although there is much interest in this direction, only the spherically symmetric case (1D) has been extensively studied. In axisymmetry (2D) fewer attempts have been made, with most of them devoted to the study of the gravitational collapse of rotating stellar cores, black hole formation, and the subsequent emission of gravitational radiation. Threedimensional simulations have only started more recently. Much effort is nowadays focused on the study of the coalescence of compact neutron star binaries (as well as the vacuum black hole binary counterpart). These theoretical investigations are driven by the emerging possibility of soon detecting gravitational waves with the different experimental efforts currently underway. The waveform catalogues resulting from timedependent hydrodynamical simulations may provide some help to data analysis groups, since the chances for detection may be enhanced through matchedfiltering techniques.
In the following, we review the status of the numerical investigations in three astrophysical scenarios all involving strong gravitational fields and, hence, relativistic physics: gravitational collapse, accretion onto black holes, and hydrodynamical evolution of neutron stars. Relativistic cosmology, another area where fundamental advances have been accomplished through numerical simulations, is not considered; the interested reader is directed to the Living Reviews article by Anninos [11] and references therein.
4.1 Gravitational collapse
The study of gravitational collapse of massive stars has been largely pursued numerically over the years. This problem was already the main motivation when May and White built the first onedimensional code [172, 173]. Such codes are designed to integrate the coupled system of Einstein field equations and relativistic hydrodynamics, sidestepping Newtonian approaches.
By browsing through the literature one realizes that the numerical study of gravitational collapse has been neatly divided between two main communities since the early investigations. First, the computational astrophysics community has traditionally focused on the physics of the (type II/Ib/Ic) supernova mechanism, i.e., collapse of unstable stellar iron cores followed by a bounce and explosion. Nowadays, numerical simulations are highly developed, as they include rotation and detailed stateoftheart microphysics (e.g., EOS and sophisticated treatments for neutrino transport). These studies are currently performed, routinely, in multidimensions with advanced HRSC schemes. Progress in this direction has been achieved, however, at the expense of accuracy in the treatment of the hydrodynamics and the gravitational field, by using Newtonian physics. In this approach the emission of gravitational radiation is computed through the quadrupole formula. For reviews of the current status in this direction see [191, 134] and references therein.
On the other hand, the numerical relativity community has been more interested, since the beginning, in highlighting those aspects of the collapse problem having to do with relativistic gravity, in particular, the emission of gravitational radiation from nonspherical collapse (see [206] for a recent Living Reviews article on gravitational radiation from gravitational collapse). Much of the effort has focused on developing reliable numerical schemes to solve the gravitational field equations and much less emphasis, if any, has been given to the details of the microphysics of the collapse. In addition, this approach has mostly considered gravitational collapse leading to black hole formation, employing relativistic hydrodynamics and gravity. Quite surprisingly, both approaches have barely interacted over the years, except for a handful of simulations in spherical symmetry. Nevertheless, numerical relativity is steadily reaching a state in which it is not unrealistic to expect a much closer interaction in the near future (see, e.g., [262, 259, 89, 260] and references therein).
4.1.1 Core collapse supernovae
However, the path to reach such conclusions has been mostly delineated from numerical simulations in one spatial dimension. Fully relativistic simulations of microphysically detailed core collapse off spherical symmetry are still absent and they might well introduce some modifications. The broad picture described above has been demonstrated in multidimensions using sophisticated Newtonian models, as is documented in [191].
Spherically symmetric simulations. May and White’s formulation and their corresponding onedimensional code formed the basis of most spherically symmetric codes built to study the collapse problem. Wilson [299] investigated the effect of neutrino transport on stellar collapse, concluding that, in contrast to previous results, heat conduction by neutrinos does not produce the ejection of material [60, 14]. The code solved the coupled set of hydrodynamic and Einstein equations, supplemented with a Boltzmann transport equation to describe the neutrino flow. Van Riper [293] used a spherically symmetric general relativistic code to study adiabatic collapse of stellar cores, considering the purely hydrodynamical bounce as the preferred explosion mechanism. The important role of general relativistic effects to produce collapses otherwise absent in Newtonian simulations was emphasized. Bruenn [49] developed a code in which the neutrino transport was handled with an approximation, the multigroup fluxlimited diffusion. Baron et al. [24] obtained a successful and very energetic explosion for a model in which the value of the incompressibility modulus of symmetric nuclear matter at zero temperature was particularly small, K _{0} ^{sym} =180 MeV (its precise value, nowadays preferred around 240 MeV [107], is still a matter of debate). Mayle et al. [174] computed neutrino spectra from stellar collapse to stable neutron stars for different collapse models using, as in [49], multigroup fluxlimited diffusion for the neutrino transport. Van Riper [294] and Bruenn [50] verified that a softer supranuclear EOS, combined with general relativistic hydrodynamics, increases the chances for a prompt explosion. In [249, 248] and [178] the neutrino transport was first handled without approximation by using a general relativistic Boltzmann equation. Whereas in [249, 248] the neutrino transport is described by moments of the general relativistic Boltzmann equation in polar slicing coordinates [22], [178] used maximal slicing coordinates. Miralles et al. [184], employing a realistic EOS based upon a HartreeFock approximation for Skyrmetype nuclear forces at densities above nuclear density, found that the explosion was favoured by soft EOS, a result qualitatively similar to that of Baron et al. [24], who used a phenomenological EOS. Swesty et al. [280] also focused on the role of the nuclear EOS in stellar collapse on prompt timescales. Contrary to previous results they found that the dynamics of the shock is almost independent of the nuclear incompressibility once the EOS is not unphysically softened as in earlier simulations (e.g., [293, 24, 294, 50, 184]). Swesty and coworkers used a finite temperature compressible liquid drop model EOS for the nulear matter component [148]. for the shock to propagate promptly to a large radius they found that the EOS must be very soft at densities just above nuclear densities, which is inconsistent with the 1.44M_{⊙} neutron star mass constraint imposed by observations of the binary pulsar PSR 1913+16.
From the above discussion it is clear that numerical simulations demonstrate a strong sensitivity of the explosion mechanism to the details of the postbounce evolution: general relativity, the nuclear EOS and the properties of the nascent neutron star, the treatment of the neutrino transport, and the neutrinomatter interaction. Recently, stateoftheart treatments of the neutrino transport have been achieved, even in the general relativistic case, by solving the Boltzmann equation in connection with the hydrodynamics, instead of using multigroup fluxlimited diffusion [177, 232, 233, 154]. The results of the few spherically symmetric simulations available show, however, that the models do not explode, neither in the Newtonian or in the general relativistic case. Therefore, computationally expensive multidimensional simulations with Boltzmann neutrino transport, able to account for convective effects, are needed to draw further conclusions on the viability of the neutrinodriven explosion mechanism.
From the numerical point of view, many of the above investigations used artificial viscosity techniques to handle shock waves. Together with a detailed description of the microphysics, the correct numerical modeling of the shock wave is the major issue in stellar collapse. In this context, the use of HRSC schemes, specifically designed to capture discontinuities, is much more recent, starting in the late 1980s with the Newtonian simulations of Fryxell et al. [103], which used an Eulerian secondorder PPM scheme (see [191] for a review of the present status). There are only a few relativistic simulations, so far restricted to spherical symmetry [129, 244, 211].
Martí et al. [162] implemented Glaister’s approximate Riemann solver [106] in a Lagrangian Newtonian hydrodynamical code. They performed a comparison of the energetics of a stellar collapse simulation using this HRSC scheme and a standard May and White code with artificial viscosity, for the same initial model, grid size and EOS. They found that the simulation employing a Godunovtype scheme produced larger kinetic energies than that corresponding to the artificial viscosity scheme, with a factor of two difference in the maximum of the infalling velocity. Motivated by this important difference, Janka et al. [136] repeated this computation with a different EOS, using a PPM secondorder Godunovtype scheme, disagreeing with Martí et al. [162]. The stateoftheart implementation of the tensorial artificial viscosity terms in [136], together with the very fine numerical grids employed (unrealistic for threedimensional simulations), could be the reason for the discrepancies.
As mentioned in Section 2.1.3, Godunovtype methods were first used to solve the equations of general relativistic hydrodynamics in [163], where the characteristic fields of the onedimensional (spherically symmetric) system were derived. The first astrophysical application was stellar collapse. In [38] the hydrodynamic equations were coupled to the Einstein equations in spherical symmetry. The field equations were formulated as a firstorder fluxconservative hyperbolic system for a harmonic gauge [39], somehow “resembling” the hydrodynamic equations. HRSC schemes were applied to both systems simultaneously (only coupled through the source terms of the equations). Results for simple models of adiabatic collapse can be found in [161, 38, 126].
The movies show that the shock is sharply resolved and free of spurious oscillations. The radius of the inner core at the time of maximum compression, at which the infall velocity is maximum (v=0.62c), is 6.3 km. The gravitational mass of the inner core at the time of maximum compression is ∼1.0M_{⊙}. The minimum value that the quantity α^{2} reaches is 0.14, which indicates the highly relativistic character of these simulations (at the surface of a typical neutron star the value of the lapse function squared is α^{2} ∼0.75).
Axisymmetric Newtonian simulations. Beyond spherical symmetry most of the existing simulations of core collapse supernova are Newtonian. Axisymmetric investigations have been performed by [190, 37, 42] using a realistic EOS and some treatment of weak interaction processes, but neglecting neutrino transport, and by [135, 188, 132, 101, 102] employing some approximate description of neutrino transport. In addition, [84, 310, 319] have performed Newtonian parameter studies of the axisymmetric collapse of rotating polytropes.
The initial data used in these calculations is taken from the 15M_{⊙} precollapse model of a blue supergiant star in [308]. The computations begin 20 ms after core bounce from a onedimensional model of [51]. This model is obtained with the onedimensional general relativistic code mentioned previously [49], which includes a detailed treatment of the neutrino physics and a realistic EOS, and which is able to follow core collapse, bounce, and the associated formation of the supernova shock. Because of neutrino cooling and energy losses due to the dissociation of iron nuclei, the shock initially stalls inside the iron core.
The movie shows how the stalling shock is “revived” by neutrinos streaming from the outer layers of the hot, nascent neutron star in the center. A negative entropy gradient builds up between the so called “gainradius”, which is the position where cooling of hot gas by neutrino emission switches into net energy gain by neutrino absorption, and the shock further out. Convective instabilities therefore set in, which are characterized by large rising blobs of neutrinoheated matter and cool, narrow downflows. The convective energy transport increases the efficiency of energy deposition in the postshock region by transporting heated material near the shock and cooler matter near the gain radius where the heating is strongest. The freshly heated matter then rises again while the shock is distorted by the upward streaming bubbles. The reader is addressed to [138] for a more detailed description of a more energetic initial model.
Axisymmetric relativistic simulations. Previous investigations in general relativistic rotational core collapse were mainly concerned with the question of black hole formation under idealized conditions (see Section 4.1.2), but none of those studies has really addressed the problem of supernova core collapse, which proceeds from white dwarf densities to neutron star densities and involves core bounce, shock formation, and shock propagation.
Wilson [301] first computed neutron star bounces of Γ=2 polytropes, and measured the gravitational wave emission. The initial configurations were either prolate or slightly aspherical due to numerical errors of an otherwise spherical collapse.
More than twenty years later, and with no other simulations in between, the first comprehensive numerical study of relativistic rotational supernova core collapse in axisymmetry has been performed by Dimmelmeier et al. [64, 65, 66, 67], who computed the gravitational radiation emitted in such events. The Einstein equations were formulated using the socalled conformally flat metric approximation [304]. Correspondingly, the hydrodynamic equations were cast as the firstorder fluxconservative hyperbolic system described in Section 2.1.3. Details of the methodology and of the numerical code are given in [66].
Dimmelmeier et al. [67] have simulated the evolution of 26 models in both Newtonian and relativistic gravity. The initial configurations are differentially rotating relativistic 4/3polytropes in equilibrium, which have a central density of 10^{10} g cm^{3}. Collapse is initiated by decreasing the adiabatic index to some prescribed fixed value Γ_{1} with 1.28 ≤ Γ_{1} ≤ 1.325. The EOS consists of a polytropic and a thermal part for a more realistic treatment of shock waves. Any microphysics, such as electron capture and neutrino transport, is neglected. The simulations show that the three different types of rotational supernova core collapse (and their associated gravitational waveforms) identified in previous Newtonian simulations by Zwerger and Müller [319] — regular collapse, multiple bounce collapse, and rapid collapse — are also present in relativistic gravity. However, rotational core collapse with multiple bounces is only possible in a much narrower parameter range in general relativity. Relativistic gravity has, furthermore, a qualitative impact on the dynamics: If the density increase due to the deeper relativistic potential is sufficiently large, a collapse that is stopped by centrifugal forces at subnuclear densities (and thus undergoes multiple bounces) in a Newtonian simulation becomes a regular, single bounce collapse in relativistic gravity. Such collapse type transitions have important consequences for the maximum gravitational wave signal amplitude. Moreover, in several of the relativistic models discussed in [67], the rotation rate of the compact remnant exceeds the critical value at which Maclaurin spheroids become secularly, and in some cases even dynamically, unstable against triaxial perturbations.

Threedimensional simulations. To date, there are no threedimensional relativistic simulations of gravitational collapse in the context of supernova core collapse yet available. All the existing computations have employed Newtonian physics. This situation, however, might change in the near future, as very recently the first fully relativistic threedimensional simulations of gravitational collapse leading to black hole formation have been accomplished, for rapidly rotating, supermassive neutron stars [262] and supermassive stars [265] (see Section 4.1.2), for the headon collision of two neutron stars [183], and for the coalescence of neutron star binaries [258, 266, 267] (see Section 4.3).
Concerning Newtonian studies, Bonazzola and Marck [42] performed pioneering threedimensional simulations, using pseudospectral methods that are very accurate and free of numerical or intrinsic viscosity. They confirmed the low amount of energy radiated in gravitational waves regardless of the initial conditions of the collapse: axisymmetric, rotating or tidally deformed (see also [190]). This result only applies to the prebounce phase of the supernova collapse as the simulations did not consider shock propagation after bounce. More recently, [234, 48] have extended the work of [319] studying, with two different PPM hydrodynamics codes, the dynamical growth of nonaxisymmetric (bar mode) instabilities appearing in rotational postcollapse cores. A relativistic extension has been performed by [261] (see Section 4.3.1).
4.1.2 Black hole formation
Apart from a few onedimensional simulations, most numerical studies of general relativistic gravitational collapse leading to black hole formation have used Wilson’s formulation of the hydrodynamics equations and finite difference schemes with artificial viscosity. The use of conservative formulations and HRSC schemes to study black hole formation, both in two and three dimensions, began rather recently.
Spherically symmetric simulations. Van Riper [293], using a (Lagrangian) May and White code, analyzed the mass division between the formation of a neutron star or a black hole after gravitational collapse. For the typical (cold) EOS used, the critical state was found to lie between the collapses of 1.95M_{⊙} and 1.96M_{⊙} cores.
In [255], a onedimensional code based on Wilson’s hydrodynamical formulation was used to simulate a general relativistic (adiabatic) collapse to a black hole. The fluid equations were finitedifferenced in a mixed EulerianLagrangian form with the introduction of an arbitrary “grid velocity” to ensure sufficient resolution throughout the entire collapse. The Einstein equations were formulated following the ADM equations. Isotropic coordinates and a maximal timeslicing condition were used to avoid the physical singularity once the black hole forms. Due to the nondynamical character of the gravitational field in the case of spherical symmetry (i.e., the metric variables can be computed at every time step solving elliptic equations), the code developed by [255] could follow relativistic configurations for many collapse timescales Δt》GM/c^{3} after the initial appearance of an event horizon.
A Lagrangian scheme based on the Misner and Sharp [185] formulation for spherically symmetric gravitational collapse (as in [293]) was developed by Miller and Motta [181] and later by Baumgarte et al. [26]. The novelty of these codes was the use of an outgoing null coordinate u (an “observertime” coordinate, as suggested previously by [124]) instead of the usual Schwarzschild time t appearing in the Misner and Sharp equations. Outgoing null coordinates are ideally suited to study black hole formation as they never cross the black hole event horizon. In these codes, the HernándezMisner equations [124] (or, alternatively, the MisnerSharp equations) were solved by an explicit finite difference scheme similar to the one used by [293]. In [181], the collapse of an unstable polytrope to a black hole was first achieved using null slicing. In [26], the collapse of a 1.4M_{⊙} polytrope with Δ=4/3 was compared to the result of [293], using the MisnerSharp equations and finding a 10% agreement. This work showed numerically that the use of a retarded time coordinate allows for stable evolutions after the black hole has formed. The Lagrangian character of both codes has prevented their multidimensional extension.
Linke et al. [156] analyzed the gravitational collapse of supermassive stars in the range 5×10^{5}M_{⊙}−10^{9}M_{⊙}. In the same spirit as in Refs. [181, 26], the coupled system of Einstein and fluid equations was solved employing coordinates adapted to a foliation of the spacetime with outgoing null hypersurfaces. From the computed neutrino luminosities, estimates of the energy deposition by νν̄annihilation were obtained. Only a small fraction of this energy is deposited near the surface of the star, where it could cause the ultrarelativistic flow believed to be responsible for GRBs. However, the simulations show that for collapsing supermassive stars with masses larger than 5×10^{5}M_{⊙}, the energy deposition is at least two orders of magnitude too small to explain the energetics of observed longduration bursts at cosmological redshifts.

Axisymmetric simulations. Beyond spherical symmetry the investigations of black hole formation started with the work of Nakamura [193], who first simulated general relativistic rotating stellar collapse. He adopted the (2+1)+1 formulation of the Einstein equations in cylindrical coordinates and introduced regularity conditions to avoid divergence behavior at coordinate singularities (the plane z=0) [195]. The equations were finitedifferenced using the donor cell scheme plus FriedrichsLax type viscosity terms. Nakamura used a “hypergeometric” slicing condition (which reduces to maximal slicing in spherical symmetry), which prevents the grid points from hitting the singularity if a black hole forms. The simulations could track the evolution of the collapse of a 10M_{⊙} “core” of a massive star with different amounts of rotational energy and an initial central density of 3−10^{13}g cm^{3}, up to the formation of a rotating black hole. However, the resolution affordable due to limitations in computer resources (42×42 grid points) was not high enough to compute the emitted gravitational radiation. Note that the energy emitted in gravitational waves is very small compared to the total rest mass energy, which makes its accurate numerical computation very challenging. In subsequent works, Nakamura [194] (see also [197]) considered a configuration consisting of a neutron star (M=1.09M_{⊙}, ρ_{c}=10^{15}g cm^{3}) with an accreted envelope of 0.81M_{⊙}, which was thought to mimic mass fallback in a supernova explosion. Rotation and infall velocity were added to such configurations, simulating the evolution depending on the prescribed rotation rates and rotation laws.
Bardeen, Stark, and Piran, in a series of papers [22, 276, 228, 277], studied the collapse of rotating relativistic (Γ=2) polytropic stars to black holes, succeeding in computing the associated gravitational radiation. The field and hydrodynamic equations were formulated using the 3+1 formalism, with radial gauge and a mixture of (singularity avoiding) polar and maximal slicing. The initial model was a spherically symmetric relativistic polytrope in equilibrium of mass M, central density \(1.9 \times {10^{15}}{(M/{M_ \odot })^{  2}}\), and radius \(6GM/{c^2} = 8.8 \times {10^5}M/{M_ \odot }\) cm. Rotational collapse was induced by lowering the pressure in the initial model by a prescribed fraction, and by simultaneously adding an angular momentum distribution that approximates rigidbody rotation. Their parameter space survey showed how black hole formation (of the Kerr class) occurs only for angular momenta less than a critical value. The numerical results also demonstrated that the gravitational wave emission from axisymmetric rotating collapse to a black hole was \(\Delta E/M{c^2} < 7 \times {10^{  4}}\), and that the general waveform shape was nearly independent of the details of the collapse.
Evans [79], building on previous work by Dykema [74], also studied the axisymmetric gravitational collapse problem for nonrotating matter configurations. His numerical scheme to integrate the matter fields was more sophisticated than in previous approaches, as it included monotonic upwind reconstruction procedures and flux limiters. Discontinuities appearing in the flow solution were stabilized by adding artificial viscosity terms in the equations, following Wilson’s approach.
Most of the axisymmetric codes discussed so far adopted spherical polar coordinates. Numerical instabilities are encountered at the origin (r=0) and at the polar axis (θ=0, π), where some fields diverge due to the coordinate singularities. Evans made important contributions toward regularizing the gravitational field equations in such situations [79]. These coordinate problems have deterred researchers from building threedimensional codes in spherical coordinates. In this case, Cartesian coordinates are adopted which, despite the desired property of being everywhere regular, present the important drawback of not being adapted to the topology of astrophysical systems. As a result, this has important consequences on the grid resolution requirements. The only extension of an axisymmetric 2+1 code in spherical coordinates to three dimensions has been accomplished by Stark [275], although no applications in relativistic astrophysics have yet been presented.
Recently, Alcubierre et al. [6] proposed a method (“cartoon”) that does not suffer from stability problems at coordinate singularities and in which, in essence, Cartesian coordinates are used even for axisymmetric systems. Using this method, Shibata [259] investigated the effects of rotation on the criterion for prompt adiabatic collapse of rigidly and differentially rotating (Γ=2) polytropes to a black hole. Collapse of the initial approximate equilibrium models (computed by assuming a conformally flat spatial metric) was induced by a pressure reduction. In [259] it was shown that the criterion for black hole formation depends strongly on the amount of angular momentum, but only weakly on its (initial) distribution. Shibata also studied the effects of shock heating using a gammalaw EOS, concluding that it is important in preventing prompt collapse to black holes in the case of large rotation rates. Using the same numerical approach, Shibata and Shapiro [265] have recently studied the collapse of a uniformly rotating supermassive star in general relativity. The simulations show that the star, initially rotating at the massshedding limit, collapses to form a supermassive Kerr black hole with a spin parameter of ∼0.75. Roughly 90% of the mass of the system is contained in the final black hole, while the remaining matter forms a disk orbiting around the hole.

Threedimensional simulations. Hydrodynamical simulations of the collapse of supermassive (Γ=2) uniformly rotating neutron stars to rotating black holes, using the code discussed in Section 3.3.1, are presented in [262]. The simulations show no evidence for massive disk formation or outflows, which can be related to the moderate initial compactness of the stellar models (R/M∼6). A proofofprinciple of the capabilities of the code discussed in Section 3.3.2 to study black hole formation is presented in [89], where the gravitational collapse of a spherical, unstable, relativistic polytrope is discussed. Similar tests with differentially rotating polytropes are given in [73] for a recent artificial viscositybased, threedimensional general relativistic hydrodynamics code.
4.1.3 Critical collapse
Critical phenomena in gravitational collapse were first discovered numerically by Choptuik in spherically symmetric simulations of the massless KleinGordon field minimally coupled to gravity [56]. Since then, critical phenomena arising at the threshold of black hole formation have been found in a variety of physical systems, including the perfect fluid model (see [114] for a review).
Evans and Coleman [81] first observed critical phenomena in the spherical collapse of radiation fluid, i.e., a fluid obeying an EOS \(p = \left( {\Gamma  1} \right)\rho \left( {1 + \varepsilon } \right)\) with Γ=4/3 and ε≫1. The threshold of black hole formation was found for a critical exponent Γ∼0.36, in close agreement with that obtained in scalar field collapse [56]. Their study used Schwarzschild coordinates in radial gauge and polar slicing, and the hydrodynamic equations followed Wilson’s formulation. Subsequently, Maison [159], using a linear stability analysis of the critical solution, showed that the critical exponent varies strongly with the parameter κ≡Γ−1 of the EOS. More recently, Neilsen and Choptuik [203], using a conservative form of the hydrodynamic equations and HRSC schemes, revisited this problem. The use of a conservative formulation and numerical schemes well adapted to describe ultrarelativistic flows [204] made it possible to find evidence of the existence of critical solutions for large values of the adiabatic index Γ(1.89 ≤Γ≤2), extending the previous upper limit.
4.2 Accretion onto black holes
The study of relativistic accretion and black hole astrophysics is a very active field of research, both theoretically and observationally (see, e.g., [32] and references therein). On the one hand, advances in satellite instrumentation, such as the Rossi XRay Timing Explorer (RXTE) and the Advanced Satellite for Cosmology and Astrophysics (ASCA), are greatly stimulating and guiding theoretical research on accretion physics. The discovery of kHz quasiperiodic oscillations in lowmass Xray binaries extends the frequency range over which these oscillations occur into timescales associated with the innermost regions of the accretion process (for a review, see [288]). Moreover, in extragalactic sources, spectroscopic evidence (broad iron emission lines) increasingly points to (rotating) black holes being the accreting central objects [282, 144, 47]. Thick accretion discs (or tori) are probably orbiting the central black holes of many astrophysical objects such as quasars and other active galactic nuclei (AGNs), some Xray binaries, and microquasars. In addition, they are believed to exist at the central engine of cosmic GRBs.
Disk accretion theory is primarily based on the study of (viscous) stationary flows and their stability properties through linearized perturbations thereof. A wellknown example is the solution consisting of isentropic constant angular momentum tori, unstable to a variety of nonaxisymmetric global modes, discovered by Papaloizou and Pringle [223] (see [18] for a review of instabilities in astrophysical accretion disks). Since the pioneering work by Shakura and Sunyaev [253], thin disk models, parametrized by the socalled αviscosity, in which the gas rotates with Keplerian angular momentum which is transported radially by viscous stress, have been applied successfully to many astronomical objects. The thin disk model, however, is not valid for high luminosity systems, as it is unstable at high mass accretion rates. In this regime, Abramowicz et al. [3] found the socalled slim disk solution, which is stable against viscous and thermal instabilities. More recently, much theoretical work has been devoted to the problem of slow accretion, motivated by the discovery that many galactic nuclei are underluminous (e.g., NGC 4258). Proposed accretion models involve the existence of advectiondominated accretion flows (ADAF solution; see, e.g., [202, 200]) and advectiondominated inflowoutflow solutions (ADIOS solution [33]). The importance of convection for low values of the viscosity parameter α is currently being discussed in the socalled convectiondominated accretion flow (CDAF solution; see [130] and references therein). The importance of magnetic fields and their consequences for the stability properties of this solution are critically discussed in [17].
For a wide range of accretion problems, the Newtonian theory of gravity is adequate for the description of the background gravitational forces (see, e.g., [98]). Extensive experience with Newtonian astrophysics has shown that explorations of the relativistic regime benefit from the use of model potentials. In particular, the PaczyńskiWiita pseudoNewtonian potential for a Schwarzschild black hole [217], gives approximations of general relativistic effects with an accuracy of 10–20% outside the marginally stable radius (at r=6M, or three times the Schwarzschild radius). Nevertheless, for comprehensive numerical work a threedimensional formalism is required, able to cover also the maximally rotating hole. In rotating spacetimes the gravitational forces cannot be captured fully with scalar potential formalisms. Additionally, geometric regions such as the ergosphere would be very hard to model without a metric description. Whereas the bulk of emission occurs in regions with almost Newtonian fields, only the observable features attributed to the inner region may crucially depend on the nature of the spacetime.
In the following, we present a summary of representative timedependent accretion simulations in relativistic hydrodynamics. We concentrate on multidimensional simulations. In the limit of spherical accretion, exact stationary solutions exist for both Newtonian gravity [43] and relativistic gravity [179]. Such solutions are commonly used to calibrate timedependent hydrodynamical codes, by analyzing whether stationarity is maintained during a numerical evolution [123, 163, 78, 244, 21].
4.2.1 Disk accretion
Pioneering numerical efforts in the study of black hole accretion [300, 123, 120, 121] made use of the socalled frozen star paradigm of a black hole. In this framework, the time “slicing” of the spacetime is synchronized with that of asymptotic observers far from the hole. Within this approach, Wilson [300] first investigated numerically the timedependent accretion of inviscid matter onto a rotating black hole. This was the first problem to which his formulation of the hydrodynamic equations, as presented in Section 2.1.2, was applied. Wilson used an axisymmetric hydrodynamical code in cylindrical coordinates to study the formation of shock waves and the Xray emission in the strongfield regions close to the black hole horizon, and was able to follow the formation of thick accretion disks during the simulations.
Wilson’s formulation has been extensively used in timedependent numerical simulations of thick disk accretion. In a system formed by a black hole surrounded by a thick disk, the gas flows in an effective (gravitational plus centrifugal) potential, whose structure is comparable to that of a close binary. The Roche torus surrounding the black hole has a cusplike inner edge located at the Lagrange point L_{1}, where mass transfer driven by the radial pressure gradient is possible [1]. In [123] (see also [120]), Hawley and collaborators studied, in the test fluid approximation and axisymmetry, the evolution and development of nonlinear instabilities in pressuresupported accretion disks formed as a consequence of the spiraling infall of fluid with some amount of angular momentum. The code used explicit secondorder finite difference schemes with a variety of choices to integrate the transport terms of the equations (i.e., those involving changes in the state of the fluid at a specific volume in space). The code also used a staggered grid (with scalars located at the cell centers and vectors at the cell boundaries) for its suitability to finite difference the transport equations in a stable numerical way. Discontinuous solutions were stabilized with artificial viscosity terms.
With a threedimensional extension of the axisymmetric code of Hawley et al. [122, 123], Hawley [121] studied the global hydrodynamic nonaxisymmetric instabilities in thick, constant angular momentum accretion gas tori orbiting around a Schwarzschild black hole. Such simulations showed that the PapaloizuPringle instability saturates in a strong spiral pressure wave, not in turbulence. In addition, the simulations confirmed that accretion flows through the torus could reduce and even halt the growth of the global instability. Extensions to Kerr spacetimes have been recently reported in [62].
Yokosawa [313, 314], also using Wilson’s formulation, studied the structure and dynamics of relativistic accretion disks and the transport of energy and angular momentum in magnetohydrodynamical accretion onto a rotating black hole. In his code, the hydrodynamic equations are solved using the FluxCorrectedTransport (FCT) scheme [46] (a secondorder fluxlimiter method that avoids oscillations near discontinuities by reducing the magnitude of the numerical flux), and the magnetic induction equation is solved using the constrained transport method [80]. The code contains a parametrized viscosity based on the αmodel [253]. The simulations revealed different flow patterns inside the marginally stable orbit, depending on the thickness h of the accretion disk. For thick disks with h ≥ 4r_{h}, r_{h} being the radius of the event horizon, the flow pattern becomes turbulent.
Igumenshchev and Beloborodov [131] have performed twodimensional relativistic hydrodynamical simulations of inviscid transonic disk accretion onto a Kerr black hole. The hydrodynamical equations follow Wilson’s formulation, but the code avoids the use of artificial viscosity. The advection terms are evaluated with an upwind algorithm that incorporates the PPM scheme [58] to compute the fluxes. Their numerical work confirms analytic expectations: (i) The structure of the innermost disk region strongly depends on the black hole spin, and (ii) the mass accretion rate goes as \(\dot M\propto {\left( {\Delta W} \right)^{\Gamma /(\Gamma  1)}}\), ΔW being the potential barrier between the inner edge of the disk and the cusp, and Γ the adiabatic index.
Furthermore, thick accretion disks orbiting black holes may be subjected to the socalled runaway instability, as first proposed by Abramowicz et al. [2]. Starting with an initial disk βlling its Roche lobe, so that mass transfer is possible through the cusp located at the L_{1} Lagrange point, two evolutions are feasible when the mass of the black hole increases: (i) Either the cusp moves inwards towards the black hole, which slows down the mass transfer, resulting in a stable situation, or (ii) the cusp moves deeper inside the disk material. In the latter case, the mass transfer speeds up, leading to the runaway instability. This instability, whose existence is still a matter of debate (see, e.g., [87] and references therein), is an important issue for most models of cosmic GRBs, where the central engine responsible for the initial energy release is such a system consisting of a thick disk surrounding a black hole.
In [87], Font and Daigne presented timedependent simulations of the runaway instability of constant angular momentum thick disks around black holes. The study was performed using a fully relativistic hydrodynamics code based on HRSC schemes and the conservative formulation discussed in Section 2.1.3. The selfgravity of the disk was neglected and the evolution of the central black hole was assumed to be that of a sequence of Schwarzschild black holes of varying mass. The black hole mass increase is determined by the mass accretion rate across the event horizon. In agreement with previous studies based on stationary models, [87] found that by allowing the mass of the black hole to grow the disk becomes unstable. For all disktohole mass ratios considered (between 1 and 0.05), the runaway instability appears very fast on a dynamical timescale of a few orbital periods (1→100), typically a few 10 ms and never exceeding 1 s, for a 2.5M_{⊙} black hole and a large range of mass fluxes (ṁ λ 103M_{⊙} s1).
Extensions of this work to marginally stable (or even stable) constant angular momentum disks are reported in Zanotti et al. [239] (animations can be visualized at the website listed in [236]). Furthermore, recent simulations with nonconstant angular momentum disks and rotating black holes [86], show that the instability is strongly suppressed when the specific angular momentum of the disk increases with the radial distance as a power law, \(l\;\propto \;{r^\alpha }\). Values of α much smaller than the one corresponding to the Keplerian angular momentum distribution suffice to supress the instability.
4.2.2 Jet formation
Numerical simulations of relativistic jets propagating through progenitor stellar models of collapsars have been presented in [9]. The collapsar scenario, proposed by [307], is currently the most favoured model for explaining long duration GRBs. The simulations performed by [9] employ the threedimensional code GENESIS [8] with a 2D spherical grid and equatorial plane symmetry. The gravitational field of the black hole is described by the Schwarzschild metric, and the relativistic hydrodynamic equations are solved in the test fluid approximation using a Godunovtype scheme. Aloy et al. [9] showed that the jet, initially formed by an ad hoc energy deposition of a few 10^{50} erg s^{1} within a 30° cone around the rotation axis, reaches the surface of the collapsar progenitor intact, with a maximum Lorentz factor of ∼33.
The most promising processes for producing relativistic jets like those observed in AGNs, microquasars, and GRBs involve the hydromagnetic centrifugal acceleration of material from the accretion disk [34], or the extraction of rotational energy from the ergosphere of a Kerr black hole [225, 36]. Koide and coworkers have performed the first MHD simulations of jet formation in general relativity [141, 140, 142]. Their code uses the 3+1 formalism of general relativistic conservation laws of particle number, momentum, and energy, and Maxwell equations with infinite electric conductivity. The MHD equations are numerically solved in the test fluid approximation (in the background geometry of Kerr spacetime) using a finite difference symmetric scheme [61]. The Kerr metric is described in BoyerLindquist coordinates, with a radial tortoise coordinate to enhance the resolution near the horizon.
We note that, recently, van Putten and Levinson [292] have considered, theoretically, the evolution of an accretion torus in suspended accretion, in connection with GRBs. These authors claim that the formation of baryonpoor outflows may be associated with a dissipative gap in a differentially rotating magnetic flux tube supported by an equilibrium magnetic moment of the black hole. Numerical simulations of nonideal MHD, incorporating radiative processes, are necessary to validate this picture.
4.2.3 Wind accretion
The term “wind” or hydrodynamic accretion refers to the capture of matter by a moving object under the effects of the underlying gravitational field. The canonical astrophysical scenario in which matter is accreted in such a nonspherical way was suggested originally by Bondi, Hoyle, and Lyttleton [125, 44], who studied, using Newtonian gravity, the accretion onto a gravitating point mass moving with constant velocity through a nonrelativistic gas of uniform density. The matter flow inside the accretion radius, after being decelerated by a conical shock, is ultimately captured by the central object. Such a process applies to describe mass transfer and accretion in compact Xray binaries, in particular in the case in which the donor (giant) star lies inside its Roche lobe and loses mass via a stellar wind. This wind impacts on the orbiting compact star forming a bowshaped shock front around it. This process is also believed to occur during the common envelope phase in the evolution of a binary system.
Numerical simulations have extended the simplified analytic models and have helped to develop a thorough understanding of the hydrodynamic accretion scenario, in its fully threedimensional character (see, e.g., [245, 27] and references therein). The numerical investigations have revealed the formation of accretion disks and the appearance of nontrivial phenomena such as shock waves and tangential (flippflop) instabilities.
Most of the existing numerical work has used Newtonian hydrodynamics to study the accretion onto nonrelativistic stars [245]. For compact accretors such as neutron stars or black holes, the correct numerical modeling requires a general relativistic hydrodynamical description. Within the relativistic, frozen star framework, wind accretion onto “moving” black holes was first studied in axisymmetry by Petrich et al. [226]. In this work, Wilson’s formulation of the hydrodynamic equations was adopted. The integration algorithm was borrowed from [277] with the transport terms finitedifferenced following the prescription given in [123]. An artificial viscosity term of the form Q=aρ(Δv)^{2}, with a a being constant, was added to the pressure terms of the equations in order to stabilize the numerical scheme in regions of sharp pressure gradients.
An extensive survey of the morphology and dynamics of relativistic wind accretion past a Schwarzschild black hole was later performed by [91, 90]. This investigation differs from [226] in both the use of a conservative formulation for the hydrodynamic equations (see Section 2.1.3) and the use of advanced HRSC schemes. Axisymmetric computations were compared to [226], finding major differences in the shock location, opening angle, and accretion rates of mass and momentum. The reasons for the discrepancies are related to the use of different formulations, numerical schemes, and grid resolution, much higher in [91, 90]. Nonaxisymmetric twodimensional studies, restricted to the equatorial plane of the black hole, were discussed in [90], motivated by the nonstationary patterns found in Newtonian simulations (see, e.g., [27]). The relativistic computations revealed that initially asymptotic uniform flows always accrete onto the hole in a stationary way that closely resembles the previous axisymmetric patterns.
4.2.4 Gravitational radiation
Semianalytical studies of finitesized collections of dust, shaped in the form of stars or shells and falling isotropically onto a black hole are available in the literature [198, 118, 256, 213, 227]. These investigations approximate gravitational collapse by a dust shell of mass m falling into a Schwarzschild black hole of mass M ≫ m. These studies have shown that for a fixed amount of infalling mass, the gravitational radiation efficiency is reduced compared to the point particle limit (ΔE ∼ 0.0104m^{2}/M), due to cancellations of the emission from distinct parts of the extended object.
In [220], such conclusions were corroborated with numerical simulations of the gravitational radiation emitted during the accretion process of an extended object onto a black hole. The firstorder deviations from the exact black hole geometry were approximated using curvature perturbations induced by matter sources whose nonlinear evolution was integrated using a (nonlinear) hydrodynamics code (adopting the conservative formulation of Section 2.1.3 and HRSC schemes). All possible types of curvature perturbations are captured in the real and imaginary parts of the Weyl tensor scalar (see, e.g., [55]). In the framework of the NewmanPenrose formalism, the equations for the perturbed Weyl tensor components decouple, and when written in the frequency domain, they even separate [285]. Papadopoulos and Font [220] used the limiting case for Schwarzschild black holes, i.e., the inhomogeneous BardeenPress equation [23]. The simulations showed the gradual excitation of the black hole quasinormal mode frequency by sufficiently compact shells.
Onedimensional numerical simulations of a selfgravitating perfect fluid accreting onto a black hole were presented in [222], where the effects of mass accretion during the gravitational wave emission from a black hole of growing mass were explored. Using the conservative formulation outlined in Section 2.2.2 and HRSC schemes, Papadopoulos and Font [222] performed the simulations adopting an ingoing null foliation of a spherically symmetric black hole spacetime [221]. Such a foliation penetrates the black hole horizon, allowing for an unambiguous numerical treatment of the inner boundary. The essence of nonspherical gravitational perturbations was captured by adding the evolution equation for a minimally coupled massless scalar field to the (characteristic) Einsteinperfect fluid system. The simulations showed the familiar dampedoscillatory radiative decay, with both decay rate and frequencies being modulated by the mass accretion rate. Any appreciable increase in the horizon mass during the emission reflects on the instantaneous signal frequency f, which shows a prominent negative branch in the ḟ(f) evolution diagram. The features of the frequency evolution pattern reveal key properties of the accretion event, such as the total accreted mass and the accretion rate.
Recently, Zanotti et al. [317] have performed hydrodynamical simulations of constant angular momentum thick disks (of typical neutron star densities) orbiting a Schwarzschild black hole. Upon the introduction of perturbations, these systems either become unstable to the runaway instability [87] or exhibit a regular oscillatory behaviour resulting in a quasiperiodic variation of the accretion rate as well as of the mass quadrupole (animations can be visualized at the website listed in [236]). Zanotti et al. [317] have found that the latter is responsible for the emission of intense gravitational radiation whose amplitude is comparable or larger than the one expected in stellar core collapse. The strength of the gravitational waves emitted and their periodicity are such that signaltonoise ratios \(\sim {\mathcal O}(1)  {\mathcal O}(10)\) can be reached for sources at 20 Mpc or 10 Kpc, respectively, making these new sources of gravitational waves potentially detectable.
4.3 Hydrodynamical evolution of neutron stars
The numerical investigation of many interesting astrophysical processes involving neutron stars, such as the rotational evolution of protoneutron stars (which can be affected by a dynamical bar mode instability and by the ChandrasekharFriedmanSchutz instability) or the gravitational radiation from unstable pulsation modes or, more importantly, from the catastrophic coalescence and merger of neutron star compact binaries, requires the ability of accurate, longterm hydrodynamical evolutions employing relativistic gravity. These scenarios are receiving increasing attention in recent years [263, 168, 96, 183, 258, 262, 266, 97, 89, 260].
4.3.1 Pulsations of relativistic stars
The use of relativistic hydrodynamical codes to study the stability properties of neutron stars and to compute mode frequencies of oscillations of such objects has increased in recent years (see, e.g., the Living Reviews article by Stergioulas [278] and references therein). An obvious advantage of the hydrodynamical approach is that it includes, by construction, nonlinear effects, which are important in situations where the linearized equations (commonly employed in calculations of modefrequencies of pulsating stars) break down. In addition, due to the current status of hydrodynamics codes, the computation of mode frequencies in rapidly rotating relativistic stars might be easier to achieve through nonlinear numerical evolutions than by using perturbative computations (see, e.g., the results in [89, 260]).
Hydrodynamical evolutions of polytropic models of spherical neutron stars can be used as testbed computations for multidimensional codes. Representative examples are the simulations by [111], with pseudospectral methods, and by [244] with HRSC schemes. These investigations adopted radialgauge polarslicing coordinates in which the general relativistic equations are expressed in a simple way that resembles Newtonian hydrodynamics. Gourgoulhon [111] used a numerical code to detect, dynamically, the zero value of the fundamental mode of a neutron star against radial oscillations. Romero et al. [244] highlighted the accuracy of HRSC schemes by finding, numerically, a change in the stability behavior of two slightly different initial neutron star models: For a given EOS, a model with mass 1.94532M_{⊙} is stable and a model of 1.94518M_{⊙} is unstable. More recently, in [274] a method based on the nonlinear evolution of deviations from a background stationaryequilibrium star was applied to study nonlinear radial oscillations of a neutron star. The accuracy of the approach permitted a detailed investigation of nonlinear features such as quadratic and higher order modecoupling and nonlinear transfer of energy.
Axisymmetric pulsations of rotating neutron stars can be excited in several scenarios, such as core collapse, crust and core quakes, and binary mergers, and they could become detectable either via gravitational waves or highenergy radiation. An observational detection of such pulsations would yield valuable information about the EOS of relativistic stars. As a first step towards the study of pulsations of rapidly rotating relativistic stars, Font, Stergioulas, and Kokkotas [97] developed an axisymmetric numerical code that integrates the equations of general relativistic hydrodynamics in a fixed background spacetime. The finite difference code is based on a stateoftheart approximate Riemann solver [70] and incorporates different second and thirdorder TVD and ENO numerical schemes. This code is capable of accurately evolving rapidly rotating stars for many rotational periods, even for stars at the massshedding limit. The test simulations reported in [97] showed that, for nonrotating stars, small amplitude oscillations have frequencies that agree to better than 1% with linear normalmode frequencies (radial and nonradial) in the socalled Cowling approximation (i.e., when the evolution of the spacetime variables is neglected). Axisymmetric modes of pulsating nonrotating stars are computed in [269], both in Cowling and fully coupled evolutions. Contrary to the 2+1 approach followed by [97], the code used in [269] evolves the relativistic stars on null spacetime foliations (see Section 2.2.2).
Until very recently (see below), the quasiradial modes of rotating relativistic stars had been studied only under simplifying assumptions, such as in the slowrotation approximation or in the relativistic Cowling approximation. An example of the latter can be found in [88], where a comprehensive study of all loworder axisymmetric modes of uniformly and rapidly rotating relativistic stars was presented, using the code developed by [97]. The frequencies of quasiradial and nonradial modes with spherical harmonic indices \(\ell = 0,1,2\) and 3 were computed through Fourier transforms of the time evolution of selected fluid variables. This was done for a sequence of appropriately perturbed stationary rotating stars, from the nonrotating limit to the massshedding limit. The frequencies of the axisymmetric modes are affected significantly by rotation only when the rotation rate exceeds about 50% of the maximum allowed. As expected, at large rotation rates, apparent mode crossings between different modes appear.
In [89], the first mode frequencies of uniformly rotating stars in full general relativity and rapid rotation were obtained, using the threedimensional code GR_ASTRO described in Section 3.3.2. Such frequencies were computed both in fixed spacetime evolutions (Cowling approximation) and in coupled hydrodynamical and spacetime evolutions. The simulations used a sequence of (perturbed) N=1, K=100 (G=c=M_{⊙}=1) polytropes of central density ρ_{c}=1.28×10^{3}, in which the rotation rate Ω varies from zero to 97% of the maximum allowed rotational frequency, Ω_{K}=0.5363×10^{4} s^{1}. The Cowling runs allowed a comparison with earlier results reported by [88], obtaining agreement at the 0.5% level. The fundamental modefrequencies and their first overtones obtained in fully coupled evolutions show a dependence on the increased rotation which is similar to the one observed for the corresponding frequencies in the Cowling approximation [88].
Relativistic hydrodynamical simulations of nonlinear rmodes are presented in [279] (see also [155] for Newtonian simulations). The gravitational radiation reactiondriven instability of the rmodes might have important astrophysical implications, provided that the instability does not saturate at low amplitudes by nonlinear effects or by dissipative mechanisms. Using a version of the @@page52GRASTRO code, Stergioulas and Font [279] found evidence that the maximum rmode amplitude in isentropic stars is of order unity. The dissipative mechanisms proposed by different authors to limit the mode amplitude include shear and bulk viscosity, energy loss to a magnetic field driven by differential rotation, shock waves, or the slow leak of the rmode energy into some short wavelength oscillation modes (see [16] and references therein). The latter mechanism would dramatically limit the rmode amplitude to a small value, much smaller than those found in the simulations of [279, 155] (see [278] for a complete list of references on the subject). Energy leak of the rmode into other fluid modes has been recently considered by [113] through Newtonian hydrodynamical simulations, finding a catastrophic decay of the amplitude only once it has grown to a value larger than that reported by [16].
The bar mode instability in differentially rotating stars in general relativity has been analyzed by [261] by means of 3+1 hydrodynamical simulations. Using the code discussed in Section 3.3.1, Shibata et al. [261] found that the critical ratio of rotational kinetic energy to gravitational binding energy for compact stars with M/R ≥ 0.1 is ∼ 0.24–0.25, slightly below the Newtonian value ∼ 0.27 for incompressible Maclaurin spheroids. All unstable stars are found to form bars on a dynamical timescale.
4.3.2 Binary neutron star coalescence
Many of the current efforts in code development in relativistic astrophysics aim to simulate the coalescence of compact binaries. Neutron star binaries are among the most promising sources of gravitational radiation to be detected by the various groundbased interferometers worldwide. The computation of the gravitational waveform during the most dynamical phase of the coalescence and plunge depends crucially on hydrodynamical, finitesize effects. This phase begins once the stars, initially in quasiequilibrium orbits of gradually smaller orbital radius (due to the emission of gravitational waves) reach the socalled innermost stable circular orbit. About ∼10^{8} years after formation of the binary system, the gravitational wave frequency enters the LIGO/VIRGO high frequency band. The final plunge of the two objects takes place on a dynamical timescale of a few ms. During the last 15 minutes or so until the stars finally merge, the frequency is inside the LIGO/VIRGO sensitivity range. About 16,000 cycles of waveform oscillation can be monitored, while the frequency gradually shifts from ∼10 Hz to ∼1 kHz. A perturbative treatment of the gravitational radiation in the quadrupole approximation is valid as long as M/R 《 1 and M/r 《 1 simultaneously, M being the total mass of the binary, R the neutron star radius, and r the separation of the two stars. As the stars approach each other and merge, both inequalities are less valid and therefore, fully relativistic hydrodynamical calculations become necessary.
The accurate simulation of a binary neutron star coalescence is, however, one of the most challenging tasks in numerical relativity. These scenarios involve strong gravitational fields, matter motion with (ultra)relativistic speeds, and relativistic shock waves. The numerical difficulties are exacerbated by the intrinsic multidimensional character of the problem and by the inherent complexities in Einstein’s theory of gravity, such as coordinate degrees of freedom and the possible formation of curvature singularities (e.g., collapse of matter configurations to black holes). It is thus not surprising that most of the (few) available simulations have been attempted in the Newtonian (and postNewtonian) framework (see [235] for a review). Many of these studies employ Lagrangian particle methods such as SPH, and only a few have considered (less viscous) highorder finite difference methods such as PPM [246].
Concerning relativistic simulations, Wilson’s formulation of the hydrodynamic equations (see Section 2.1.2) was used in Refs. [303, 304, 169]. Such investigations assumed a conformally flat 3metric, which reduces the (hyperbolic) gravitational field equations to a coupled set of elliptic (Poissonlike) equations for the lapse function, the shift vector, and the conformal factor. These early simulations revealed the unexpected appearance of a “binaryinduced collapse instability” of the neutron stars, prior to the eventual collapse of the final merged object. This effect was reduced, but not eliminated fully, in revised simulations [169], after Flanagan [85] pointed out an error in the momentum constraint equation as implemented by Wilson and coworkers [303, 304]. A summary of this controversy can be found in [235]. Subsequent numerical simulations with the full set of Einstein equations (see below) did not find this effect.
Nakamura and coworkers have been pursuing a programme to simulate neutron star binary coalescence in general relativity since the late 1980’s (see, e.g., [196]). The group developed a threedimensional code that solves the full set of Einstein equations and selfgravitating matter fields [214]. The equations are finitedifferenced in a uniform Cartesian grid using van Leer’s scheme [290] with TVD flux limiters. Shock waves are spread out using a tensor artificial viscosity algorithm. The hydrodynamic equations follow Wilson’s Eulerian formulation and the ADM formalism is adopted for the Einstein equations. This code has been tested by the study of the gravitational collapse of a rotating polytrope to a black hole (comparing to the axisymmetric computation of Stark and Piran [276]). Further work to achieve long term stability in simulations of neutron star binary coalescence is under way [214]. We note that the hydrodynamics part of this code is at the basis of Shibata’s code (Section 3.3.1), which has successfully been applied to simulate the binary coalescence problem (see below).
The most advanced simulations of neutron star coalescence in full general relativity are those performed by Shibata and Uryū [258, 266, 267]. Their numerical code, briefly described in Section 3.3.1, allows the longterm simulation of the coalescences of both irrotational and corotational binaries, from the innermost stable circular orbit up to the formation and ringdown of the final collapsed object (either a black hole or a stable neutron star). Their code also includes an apparent horizon finder, and can extract the gravitational waveforms emitted in the collisions. Shibata and Uryū have performed simulations for a large sample of parameters of the binary system, such as the compactness of the (equal mass) neutron stars (0.12 ≤ M/R ≤ 0.16), the adiabatic index of the Γlaw EOS (1.8 ≤ Γ ≤ 2.5), and the maximum density, rest mass, gravitational mass, and total angular momentum. The initial data correspond to quasiequilibrium states, either corotational or irrotational, the latter being more realistic from considerations of viscous versus gravitational radiation timescales. These initial data are obtained by solving the Einstein constraint equations and the equations for the gauge variables under the assumption of a conformally flat 3metric and the existence of a helical Killing vector (see [267] for a detailed explanation). The binaries are chosen at the innermost orbits for which the Lagrange points appear at the inner edge of the neutron stars, and the plunge is induced by reducing the initial angular momentum by ∼2–3%.
The comprehensive parameter space survey carried out by [258, 266, 267] shows that the final outcome of the merger depends sensitively on the initial compactness of the neutron stars before plunge. Hence, depending on the stiffness of the EOS, controlled through the value of Γ, if the total rest mass of the system is ∼1.3–1.7 times larger than the maximum rest mass of a spherical star in isolation, the end product is a black hole. Otherwise, a marginallystable massive neutron star forms. In the latter case the star is supported against selfgravity by rapid differential rotation. The star could eventually collapse to a black hole once sufficient angular momentum has dissipated via neutrino emission or gravitational radiation. The different outcome of the merger is reflected in the gravitational waveforms [267]. Therefore, future detection of highfrequency gravitational waves could help to constrain the maximum allowed mass of neutron stars. In addition, for prompt black hole formation, a disk orbiting around the black hole forms, with a mass less than 1% the total rest mass of the system. Disk formation during binary neutron star coalescence, a fundamental issue for cosmological models of short duration GRBs, is enhanced for unequal mass neutron stars, in which the less massive one is tidally disrupted during the evolution (Shibata, private communication).
To close this section we mention the work of Duez et al. [72] where, through analytic modelling of the inspiral of corotational and irrotational relativistic binary neutron stars as a sequence of quasiequilibrium configurations, the gravitational wavetrain from the last phase (a few hundred cycles) of the inspiral is computed. These authors further show a practical procedure to construct the entire wavetrain through coalescence by matching the late inspiral waveform to the one obtained by fully relativistic hydrodynamical simulations as those discussed in the above paragraphs [266]. Detailed theoretical waveforms of the inspiral and plunge similar to those reported by [72] are crucial to enhance the chances of experimental detection in conjunction with matchedfiltering techniques.
5 Additional Information
This last section of the review contains technical information concerning recent developments in the implementation of Riemannsolverbased numerical schemes in general relativistic hydrodynamics.
5.1 Riemann problems in locally Minkowskian coordinates
In [229], a procedure to integrate the general relativistic hydrodynamic equations (as formulated in Section 2.1.3), taking advantage of the multitude of Riemann solvers developed in special relativity, was presented. The approach relies on a local change of coordinates in terms of which the spacetime metric is locally Minkowskian. This procedure allows, for 1D problems, the use of the exact solution of the special relativistic Riemann problem [166].
Such a coordinate transformation to locally Minkowskian coordinates at each numerical interface assumes that the solution of the Riemann problem is the one in special relativity and planar symmetry. This last assumption is equivalent to the approach followed in classical fluid dynamics, when using the solution of Riemann problems in slab symmetry for problems in cylindrical or spherical coordinates, which breaks down near the singular points (e.g., the polar axis in cylindrical coordinates). In analogy to classical fluid dynamics, the numerical error depends on the magnitude of the Christoffel symbols, which might be large whenever huge gradients or large temporal variations of the gravitational field are present. Finer grids and improved time advancing methods will be required in those circumstances.
Following [229], we illustrate the procedure for computing the second flux integral in Equation (45), which we call \({\mathcal I}\). We begin by expressing the integral on a basis \({\mathbf{e}_{\hat \alpha }}\) with \({{\bf{e}}_{\hat 0}} \equiv {n^\mu }\) and e^{ î } forming an orthonormal basis in the plane orthogonal to \({n^\mu }\) with e_{1̂} normal to the surface \({\sum _{{x^1}}}\) and e_{2̂} and e_{3̂} tangent to that surface. The vectors of this basis verify \({\mathbf{e}_{\hat \alpha }} \cdot {\mathbf{e}_{\hat \beta }} = {\eta _{\hat \alpha \hat \beta }}\) with \({\eta _{\hat \alpha \hat \beta }}\) the Minkowski metric (in the following, caret subscripts will refer to vector components in this basis).
At this point, all the theoretical work developed in recent years on special relativistic Riemann solvers can be exploited. The quantity in parentheses in Equation (64) represents the numerical flux across \({\Sigma _{{x^1}}}\), which can now be calculated by solving the special relativistic Riemann problem defined with the values at the two sides of \({\Sigma _{{x^1}}} \) of two independent thermodynamical variables (namely, the rest mass density ρ and the specific internal energy ε) and the components of the velocity in the orthonormal spatial basis \({v^{\hat \iota }}({v^{\hat \iota }} = M_i^{\hat \iota }{v^i})\).
5.2 Characteristic fields in the 3+1 conservative Eulerian formulation of Ibáñez and coworkers
This section collects all information concerning the characteristic structure of the general relativistic hydrodynamic equations in the Eulerian formulation of Section 2.1.3. As explained in Section 3.1.2, this information is necessary in order to implement approximate Riemann solvers in HRSC finite difference schemes.
Footnotes
 1.
We note however that all codes based on the 3+1 formalism share this problem since the outer boundaries are located at finite radii. Further work on the development of more sophisticated boundary conditions is needed to solve this problem. Alternative solutions are to follow the lightcone approach developed by Winicour et al. [305] or the conformal formalism of Friedrich [99]
Notes
Acknowledgements
It is a pleasure to acknowledge José M. Ibáñez and José M. Martífor valuable suggestions and a careful reading of the manuscript. The many colleagues who kindly helped me to update the previous version of the review by providing relevant information are warmly acknowledged. I am particularly grateful to Harald Dimmelmeier, Konstantinos Kifonidis, Shin Koide, José V. Romero, Masaru Shibata, and WaiMo Suen for providing some of the figures and animations contained in the article. This research has been supported in part by the Spanish Ministerio de Ciencia y Tecnología (AYA20013490C02C01), and by a Marie Curie fellowship from the European Union (HPMFCT200101172). The author has made use of NASA’s Astrophysics Data System Abstract Service, which is gratefully acknowledged.
Supplementary material
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