Fluctuations in the diffuse X-ray background observed with Ginga

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 logN − 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 normalisation of the 2–10 keV source counts is a factor 2−3 above that derived in the softer 0.3–3.5 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 < 10 44 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.