A DEEP ROSAT SURVEY .2. OBSERVATIONS OF THE ISOTROPY OF THE 1-2-KEV X-RAY-BACKGROUND

We investigate the isotropy of the 1-2 keV X-ray background with ROSAT in order to constrain the models for its origin. The photon count distribution, P(D), is not well fitted by the source count distribution predicted using models for the luminosity function and evolution determined from identified AGN. This suggests either a steeper than expected slope for the low-luminosity branch of the AGN luminosity function, or the presence of a population of sources other than QSOs and Seyfert 1 galaxies. The auto-correlation function of the background fluctuations shows a significant (3 sigma) signal at small angular scales ( < 3 arcmin), confirming the result of Hasinger. The upper limit on this signal suggests that QSOs and Seyfert 1 galaxies can produce up to almost-equal-to 50 per cent of the residual background. Any single population that produces all of the residual X-ray background is likely to have a characteristic clustering length less than roughly 8-14 Mpc (H-0 = 50 km s-1 Mpc-1), depending on the clustering evolution.