Can soft X-ray spectra of AGN be taken as emission from accretion disks?

Abstract

Soft X-ray spectra of many Active Galactic Nuclei (AGN) show structure which suggest excess emission at low energies, mostly below 1 keV. This component may be the high energy tail of the so called blue bump which in turn may be due to the integrated emission from an accretion disk around the central black hole. ROSAT has observed a large number of AGN with unprecedented sensitivity in the energy range 0.1–2.4 keV.

Here we present results of our spectral analysis of two different samples of AGN:

• 1) QSO/Seyfert-I from the ROSAT All Sky Survey and

• 2) radio-quiet quasars from ROSAT Pointed Observations.

The ROSAT data are combined with UV Data from IUE and hard X-ray data from various hard X-ray missions. We give results on individual objects as well as on statistical properties of the samples.

The ROSAT spectra of AGN in both samples are found to generally have power law spectra steeper than the canonical value of 0.7, establishing a steep soft X-ray component. The soft X-ray component — together with UV and hard X-ray data — is then described in terms of thermal emission from a thin α-accretion disk, including Comptonization and relativistic corrections. We have developed disk models of increasing complexity. In fitting these models to the observational data constraints on the physical parameters of the systems — the mass of the black hole, the accretion rate, the viscosity and the inclination angle — are derived.

Our general conclusion is that emission from an accretion disk can in many cases account for the observed spectral features, such as the soft X-ray excess as the high energy tail of the big blue bump.