The large scale structure of the soft X - ray background III . Cosmological implications

The auto-correlation function of the X-ray background in the range of angular scales 0. ◦3–20◦ is determined using the ROSAT All-Sky Survey. In the power-law approximation, our best fit estimate is w(θ) = (0. ◦0058/θ)1.1. At the small separation end, this is substantially above previous estimates based on ROSAT pointings. The present upward revision of the correlation amplitude results from variations in the background intensity at angular scales larger than the field of view of the X-ray telescope. Various assumptions for the local clustering amplitude, clustering evolution rate and luminosity/density evolution of the sources producing the background are explored, and the predicted auto-correlation functions compared with the measured fluctuations. We show that the large amplitude of the background angular auto-correlation function is incompatible with the weak clustering amplitude observed for normal galaxies, while it is marginally compatible with some measurements of the AGN clustering amplitude at moderate redshifts.