The 2-10 KeV X-ray background dipole and its cosmological implications

The hard X-ray (>2 keV) emission of the local and distant universe as observed with the HEAO I A-2 experiment is reconsidered in the context of large-scale cosmic structure. Using all-sky X-ray samples of active galactic nuclei (AGNs) and galaxy clusters, we remove the dominant local X-ray flux from within a redshift of similar to0.02. We evaluate the dipolar and higher order harmonic structure in four X-ray colors. The estimated dipole anisotropy of the unresolved flux appears to be consistent with a combination of the Compton-Getting effect due to the Local Group motion (dipole amplitude Delta = 0.0042) and remaining large-scale structure (0.0023 less than or similar to Delta less than or similar to 0.0085), in good agreement with the expectations of cold dark matter models. The observed anisotropy does, however, also suggest a nonnegligible Galactic contribution that is more complex than current, simple models of >2 keV Galactic X-ray emission. Comparison of the soft and hard color maps with a harmonic analysis of the 1.5 keV ROSAT all-sky data qualitatively suggests that at least a third of the faint, unresolved -18" scale structure in the HEAO I A-2 data may be Galactic in origin. However, the effect on measured flux dipoles is small (less than or similar to3%). We derive an expression for dipole anisotropy and acceleration and demonstrate how the dipole anisotropy of the distant X-ray frame can constrain the amplitude of bulk motions of the universe. From observed bulk motions over a local similar to 50 h(-1) Mpc radius volume, we determine 0.14 less than or similar to Omega (0.6)(0)/b(X)(0) less than or similar to 0.59, where Omega (0), is the universal density parameter and b(X)(0) is the present-epoch bias parameter, defined as the ratio of fluctuations in the X-ray source density and the mass density.