Small-scale fluctuations and angular correlations of the X-ray background in the HEAO 1 A-2 energy band - Constraints on clustering of X-ray sources

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Abstract
Using HEAO 1 A-2 all-sky survey data we have determined the amplitude of intensity fluctuations of the extragalactic 2-10 keV X-ray background (XRB) over an effective solid angle of 1.84 deg2 and their angular correlation function on angular scales of less than 3-degrees (i.e., smaller than the detector's field of view). The observed fluctuations are congruent-to 3 sigma above those expected if the counts keep a Euclidean slope well below the limit of the Piccinotti survey, as indicated by the Einstein Observatory medium sensitivity and deep surveys. A good empirical fit to the data is obtained assuming that the integral counts in the A-2 band have a slope of 1.65(-0.07)+0.06, fully consistent with the best-fit slope of 1.72(-0.10)+0.15 obtained by Piccinotti et al. directly from source counts with the same experiment. Alternatively, the data may imply a significant clustering of extragalactic X-ray sources. However, the non-Poisson constribution of rich clusters of galaxies with clustering radii consistent with optical data (r0 less-than-or-equal-to 50-60 Mpc, H0 = 50) do not fully account for the observed signal. Clustering of AGNs could do the job; the required amplitude of the two-point correlation function depends primarily on the evolution of their luminosity and/or space density and of their clustering properties. The resulting values are consistent with optical data on quasar clustering

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