The Origin of the X-Ray Background

Abstract

Despite the hundredfold increase in sensitivity achieved with the launch of the soft X-ray Einstein Observatory, the problem of the origin of the X-ray background (XRB) is not completely resolved yet, so that the old controversy between supporters of different theories (XRB as due to discrete sources or to truly diffuse processes) is likely to continue. On the other hand, the results obtained with the Einstein Observatory have the capability of constraining quite strongly the allowed range of possibilities. Two quite different and independent approaches have been followed in order to estimate the contribution of discrete sources to the XRB. The first (Einstein Deep and Medium Surveys) has extended the observed log N — log S down to fluxes of ~3 x 10^-14 erg cm^-2s^-1 providing a firm lower limit of (26 ± 11)% for the contribution of discrete sources to the XRB. The second (observations of several hundreds of previously known extragalactic objects) has clearly shown that quasars, as a class, are the most significant contributors to the XRB. An estimate of the allowed range of their contribution is here presented, taking into account both the existence of selection effects in the observed sample of quasars and the large uncertainty about the exact shape of the optical log N-m at faint magnitudes.