The Origin of the Extragalactic X-Ray Background

Abstract

Although the shape of the spectrum in the 3–100 keV interval is suggestive of an optically thin bremsstrahlung at ~ 40 keV, it is well known that the interpretation in terms of a hot intergalactic gas (IGG) requires a rather extreme energy supply and a gas density conflicting with the baryon density upper limit derived from primordial nucleosynthesis calculations. A summary discussion of the estimated contributions from the integrated X-ray emission of known classes of extragalactic discrete sources at a reference energy of 2 keV is given. Although these estimates are still uncertain, the subtraction of a “minimum” contribution drastically modifies the 40 keV thermal shape, prima facie evidence of a hot IGG. AGNs are the main contributors. Low luminosity AGNs at redshift z = 1−2 may in fact saturate the 2 keV XRB, but their observed hard X-ray spectra are on the average unlike (much too steep) that of the XRB. This has led a number of authors to postulate new classes of sources and some exotic models which are briefly summarized. However, if a recently proposed unified scheme of AGNs holds, then the bulk of the XRB intensity can be explained independently of the observed spectral differences and with a mild cosmological evolution.