Galactic Accelerators and Acceleration Mechanisms
In order to explain the features observed in experimental data and discussed in the previous chapters, Cosmic Rays (CRs) must be accelerated by non-thermal processes. As CRs with energies up to ∼ 1020 eV have been observed, theoretical model must consider sources and processes able to accelerate particles to these extraordinary energies. In most man-made accelerators, particles are accelerated by electric fields and deflected in circular orbits by magnetic fields. The magnetic fields also ensure that the particles remain confined in the acceleration regions. In most astrophysical environments, static electric fields cannot be maintained, because the matter is in the state of a plasma. In the standard model of CR production, the bulk of CRs is believed to be accelerated in recursive stochastic mechanisms in which low-energy particles, after a large number of interactions with a shock wave, will reach high energies. In this model, supernova remnants could accelerate protons up to 1015–1016 eV, with a spectral energy index a ∼ 2, as required by experimental data. However, the standard model of galactic CR acceleration has some limitations, and particular, it fails to describe the flux above the knee. Additional models have been put forward, such as the particle acceleration through electromagnetic mechanisms associated with time-varying magnetic fields. Some peculiar galactic objects can be involved in these processes. At present, no firm experimental proof is evident for any point-like source of CRs. In the chapter, we review the possible acceleration mechanisms and the involved astrophysical sources.
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