Isolating the foreground of the X-ray background.

The foreground component of the X-ray background (XRB) can be studied by cross-correlating its intensity with galaxy catalogs. The authors computed the two-point cross-correlation function W_xg between 67 Einstein X-ray fields and the APM galaxy catalog. They detected a signal with an amplitude of W_xg = 0.04±0.01, significant at the 3.5σ level. The reality of this signal was tested with a series of control data sets. The authors discuss how this signal can be used to constrain models of the XRB.