The small-scale autocorrelation function of the X-ray background⋆

Abstract

We have studied the angular correlation function of the 1−3 keV X-ray background using data from five deep Einstein IPC fields. Only counts in the central part of the detector (0.25 deg2), where the response is nearly flat, have been considered. After removing possible (≥ 3 σ) sources, we have fitted a Gaussian to the autocorrelation function from 1 to 15 arcmin. This shows a signal below 5 arcmin, although it is not clear whether it is due to the cosmic X-ray background. On the assumption that detector effects, other than baseline (i.e. non-cosmic) counts, do not lower anisotropics, an upper limit for the autocorrelation function at 5 arcmin is ≈ 9 per cent (at 95 per cent confidence level) after estimating the contribution from non-cosmic background. The implications of this upper limit for the large-scale structure of the X-ray Universe are discussed.