On the intensity of the extragalactic X-ray background

Measurements of the intensity of the cosmic X-ray background (XRB) carried out over small solid angles are subject to spatial variations caused by the discrete nature of the XRB. This cosmic variance can account for the dispersion of XRB intensity values found within the ASCA, BeppoSAX and ROSAT missions separately. However, there are differences among the values obtained in the different missions, which are not caused by spatial fluctuations but, more likely, by systematic cross-calibration errors. Prompted by recent work which shows that the ROSAT PSPC has calibration differences from all the other missions, we compute a Bayesian estimate for the XRB intensity at 1 keV of (90 per cent confidence errors) using the ASCA and BeppoSAX data points. However, this value is still significantly larger than the HEAO-1 intensity measured over many thousands of square degrees (8 keV cm−2 s−1 sr−1 keV−1).