Astronomy and Astrophysics – Astrophysics
Scientific paper
1998-12-08
Astronomy and Astrophysics
Astrophysics
revised version, submitted to MNRAS, 6 pages, 3 figures in MNRAS LaTex style
Scientific paper
10.1046/j.1365-8711.1999.02541.x
Several models for the hard X-ray Background (XRB) suggest that it is due to the emission from heavily obscured AGN. Recent studies have revealed the presence of a new population of hard X-ray sources which must contribute significantly to the XRB. However, whether the majority of these sources are obscured AGN or some other type of object at present remains unclear. Here, we further examine the possibility that a significant fraction of the XRB comes from a population of galaxies undergoing advection-dominated accretion in the high-\mdot regime and thus produce intrinsically hard spectra. When the accretion rate is close to its critical value, above which an advection-dominated accretion flow (ADAF) no longer exists, the major contribution to X-ray emission is due to inverse Compton scattering of the soft seed photons produced by cyclo-synchrotron radiation. In this regime, the resulting ADAF spectra are relatively hard with a fairly constant X-ray spectral index \alpha \sim 0.2-0.4 and a spectral cut-off at \sim 200 keV. We show that the integrated emission from such sources can provide a good fit to the hard (> 2 keV) X-ray background, provided that the spectrum is dominated by the contribution from objects located at redshifts z \sim 2-3. The model requires most of the contribution to the XRB to be due to objects accreting at the critical rate.
Di Matteo Tiziana
Esin Ann A.
Fabian Andrea C.
Narayan Ramesh
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