Irregularities in the X-Ray Background

There are at least four irregularities now known in the distribution of the 2-10 keV X-ray background. Three of these are in the angular distribution of surface brightness: small scale (~ 25 square degrees) graininess interpreted as Poisson fluctuations in discrete source counts, a component dependent on galactic latitude and longitude which might be either a diffuse galactic emission or a galactic source population, and a residual very large scale patchiness which may be related to this galactic component or else due to irregularities in the contribution of extragalactic sources to the existing surface brightness maps. The Compton-Getting effect or large scale inhomogeneities in the universe may also contribute to this last observed structure, but such a cause cannot be deduced uniquely from data currently available. Each of these three effects has an amplitude of 2-5% and although their reality is beyond doubt, the detections are not at a high enough signal to noise ratio to allow very detailed information. Studies of source counts, and of the class properties of known X-ray emitting objects tell us about the structure of the universe in the radial coordinate. In particular, recent results from the Einstein X-ray Observatory in these areas have revealed the fourth irregularity, that the extragalactic sources evolve strongly so that half the background does arise from beyond a redshift of unity.