Open AccessOn the origin of the hard X‐ray background
Author(s) -
Di Matteo T.,
Esin A.,
Fabian A. C.,
Narayan R.
Publication year - 1999
Publication title -
monthly notices of the royal astronomical society
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.058
H-Index - 383
eISSN - 1365-2966
pISSN - 0035-8711
DOI - 10.1046/j.1365-8711.1999.02541.x
Subject(s) - physics , astrophysics , x ray background , accretion (finance) , redshift , population , galaxy , astronomy , source counts , spectral line , active galactic nucleus , sociology , demography
Several models for the hard X‐ray background (XRB) suggest that it is a result of the emission from heavily obscured AGN. Recent studies have revealed the presence of a new population of hard X‐ray sources that 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‐m˙ regime and thus produce intrinsically hard spectra. When the accretion rate is close to m˙ crit , above which an advection‐dominated accretion flow (ADAF) no longer exists, the major contribution to X‐ray emission is from 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 α∼ 0.2‐‐0.4 and a spectral cut‐off at ∼ 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 ∼ 2‐‐3. The model requires most of the contribution to the XRB to be resulting from objects accreting at m˙ crit .