The 2–10 keV X‐Ray Background Dipole and Its Cosmological Implications

Abstract

The hard X-ray (>2 keV) emission of the local and distant universe as observed with the HEAO 1 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 focal X-ray flux from within a redshift of ∼0.02. 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 Δ = 0.0042) and remaining large-scale structure (0.0023 ≲ Δ ≲ 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 1 A-2 data may be Galactic in origin. However, the effect on measured flux dipoles is small (≲3%). 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 ∼50 h-1 Mpc radius volume, we determine 0.14 ≲ Ω0.60/bx(0) ≲ 0-59, where Ω0 is the universal density parameter and bx(0) is the present-epoch bias parameter, defined as the ratio of fluctuations in the X-ray source density and the mass density.