Fluctuations in the diffuse X-ray background observed with Ginga

Abstract

We present Ginga measurements of the spatial fluctuations in the diffuse X-ray background. When combined with earlier results, the new data constrain the extragalactic log M-log S relation in the 2-10 keV energy band to a form close to the Euclidean prediction over the flux range 10-10-5 × 10-13 erg cm-2 s-1. The normalization of the 2-10 keV source counts is a factor 2-3 above that derived in the softer 0.3-0.5 keV band from the Einstein Extended Medium Sensitivity Survey, if a spectral conversion is assumed which ignores X-ray absorption intrinsic to the sources. Both this result and the spectral characteristics of the spatial fluctuations are consistent with relatively low-luminosity active galaxies (i.e. Lx< 1044 erg s-1) dominating the 2-10 keV source counts at intermediate flux levels. We also use the 'excess variance' of the fluctuations to constrain possible clustering of the underlying discrete sources.