Limits on the Small Scale Structure of the Diffuse Cosmic X-rays

Abstract

THE diffuse cosmic X-rays, according to most theories, contain a substantial contribution from sources at a redshift distance of order z = 1, and measurement of their angular distribution thus provides data of cosmological relevance^1,2. Wolfe and Burbidge² have constructed angular autocorrelation functions for X-rays arising directly or indirectly from the observed distribution of galaxies. Available data on the X-ray isotropy in the 10 to 40 keV range³ had been obtained on an angular scale of 12°. To compare with that data, Wolfe and Burbidge had to integrate their autocorrelation functions to define an effective angular scale of fluctuations, and use beam convolution techniques⁴ to extrapolate the measured upper limit X-ray fluctuations down to that angular scale. In this communication, we compare the experimentally measured autocorrelation function with the calculations of Wolfe and Burbidge and confirm their conclusions that the X-ray isotropy is not consistent with the observed superclustering, and probably not with clustering, of galaxies. If the bulk of the diffuse cosmic X-rays arise from discrete sources, then the weakest lower limit to their number is obtained by ignoring possible clustering and yields a minimum of 5 × 10⁵ objects assuming constant apparent luminosity, or 4 × 10⁶ objects assuming constant absolute luminosity.