Fluctuations in the X-ray background and the large-scale structure of the Universe

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Abstract
We analyse the small-scale angular fluctuations of the integrated X-ray background (XRB). A general model for finite-size, clustered sources is assumed in order to compute the expected fluctuations in the XRB. This framework allows us to constrain the contribution to the XRB of sources with known size and correlation function. It is shown that if QSOs have a redshift distribution similar to the ASIAGO catalogue and cluster on scales ∼10 Mpc, as suggested in recent work, they cannot produce more than 10–30 per cent of the XRB. If, on the other hand, most of the XRB is produced by a hot intergalactic medium, severe constraints on its dumpiness are placed mainly by the small-scale angular isotropy of the cosmic microwave background. We finally discuss how future observations of the XRB on scales of 1 arcmin would give upper bounds for δϱ/ϱ on scales 10–100 Mpc.

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