  Recently, we presented evidence for a cross-correlation of the WMAP satellite
map of the cosmic microwave background (CMB) and the HEAO1 satellite map of the
hard X-ray background (XRB) with a dimensionless amplitude of 0.14 +- 0.05
normalized to the product of the rms fluctuations of the CMB and XRB (Boughn &
Crittenden, 2004). Such a correlation is expected in a universe dominated by a
cosmological constant via the integrated Sachs-Wolfe (ISW) effect and the level
of the correlation observed is consistent with that predicted by the currently
favored Lambda cold dark matter model of the universe. Since this offers
independent confirmation of the cosmological model, it is important to verify
the origin of the correlation. Here we explore in detail some possible
foreground sources of the correlation. The present evidence all supports an ISW
origin.
