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 HEAO1-A2 experiment is reconsidered in the context of large scale cosmic structure.Using all-sky X-ray samples of AGN and galaxy clusters we remove the dominant local X-ray flux from within a redshift of ∼ 0.02. We evaluate the dipolar and higher order harmonic structure in 4 X-ray colours. 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 ∆ = 0.0042) and remaining large scale structure (0.0023 < ∼ ∆ < ∼ 0.0085), in good agreement with the expectations of Cold Dark Matter models.The observed anisotropy does however also suggest a non-negligible Galactic contribution which is more complex than current, simple models of > 2keV Galactic X-ray emission. Comparison of the soft and hard colour 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 HEAO1-A2 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 < ∼ Ω 0.6 0 /b x (0) < ∼ 0.59, where Ω 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.

Large-scale variations of the cosmic X-ray background and the X-ray emissivity of the local Universe

“…We note that the obtained dipole amplitude and orientation are in good agreement with the values determined from the analysis of HEAO1/A2 data (Scharf et al 2000).…”

“…Scharf et al 2000;Kocevski et al 2004). However, higher order harmonics might be created by more distant structures.…”

The harmonic power spectrum of the soft X-ray background

“…It has not been widely used in the studies of the XRB because it requires high quality observational material. Recently Scharf et al (2000) investigated the all-sky HEAO1-A2 data in the 2-10 keV band in order to estimate the XRB dipole. Due to the low resolution of the HEAO 1 data, the angular power spectrum of the XRB up to l = 20 was calculated.…”

Probing large scale homogeneity and periodicity in the LRG distribution using Shannon entropy

“…So far the most compelling evidence of isotropy comes from the near uniform temperature of the CMBR over the full sky (Penzias & Wilson 1965;Smoot et al 1992;Fixsen et al 1996). This is further supported by multitude of evidences such as the isotropy in angular distributions of radio sources (Wilson & Penzias 1967;Blake & Wall 2002), isotropy in the X-ray background (Peebles 1993; ⋆ E-mail: biswap@visva-bharati.ac.in † E-mail:suman2reach@gmail.com Wu et al 1999;Scharf et al 2000), isotropy of Gammaray bursts (Meegan et al 1992;Briggs et al 1996), isotropy in the distribution of galaxies (Marinoni et al 2012;Alonso et al 2015), isotropy in the distribution of supernovae (Gupta & Saini 2010;Lin et al 2015) and isotropy in the distribution of neutral hydrogen (Hazra & Shafieloo 2015). But merely having isotropy around us does not guarantee homogeneity of the Universe.…”