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

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 relevance1,2. 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.