Is There a Foreground to the X-ray Background?

The recent suggestion of Lahav et al.(Nature, 364, 693) that a significant fraction of the cosmic X-ray background orginates from a non-evolving population of objects traced by local galaxies is based on the cross-correlation of bright galaxy catalogs with the X-ray background intensity derived from non-imaging X-ray detectors in the 2--20 keV band. In order to test this result directly and to examine the energy-dependence of this contribution, we have undertaken a study comparing Einstein imaging proportional counter (IPC) images of the background with galaxy catalogs derived from the Cambridge APM POSS plate scans. Specifically, we have chosen sixty-three 1 deg(2) fields at high Galactic latitude observed by the Einstein IPC and extracted the complete APM catalog for each image. We then compute the zero-lag cross-correlation function for the APM galaxy counts (down to m_r ~ 20) and the X-ray intensity in cell sizes ranging from 1 to 25 arcmin(2) . The procedure is carried out both before and after excision of X-ray point sources above a selected threshold. Tests of the significance of the correlation signal are provided by two types of control experiments: 1) repeating the correlation analysis for the galaxies with X-ray data from the identical fields edited so as to minimize the extragalactic background included (by using the 0.1--1 keV band only during periods of high solar scattered flux contamination), and 2) by crosscorrelating the cosmic X-ray intensity with the stars found in the same fields. Slicing the galaxy catalog into various magnitude intervals allows us to examine directly the contribution of redshift shells from z ~ 0 to z ~ 0.25. We present limits on the correlation amplitude as a function of X-ray energy and discuss the constraints these limits place on the intrinsic spectrum of potential source populations contributing to the X-ray background. This work was supported by NASA grant NAGW-2507.