On the intensity of the extragalactic X-ray background

Abstract

Measurements of the intensity of the cosmic X-ray background (XRB) carried out over small solid angles are subject to spatial variations due to the discrete nature of the XRB. We find variations of the order of 10 per cent for solid angles under 1 deg2 and, therefore, cosmic variance often dominates over the statistical uncertainties quoted in the various estimates of the XRB intensity. Taking into account this fact, we have computed Bayesian estimates for the XRB intensity at 1 keV using different measurements of the intensity carried out with the imaging instruments onboard the ASCA, BeppoSAX and ROSAT missions. Our corresponding estimates are 9.8−1.0 +0.7 (ASCA), 11.1−2.3 +1.0 (BeppoSAX), and 12.7−1.9 +0.9 keV cm−2s−1 sr−1 keV−1 (ROSAT) (90 per cent errors).