The nature of the unresolved extragalactic soft CXB

TL;DR

This study analyzes the unresolved soft X-ray background using deep Chandra observations, revealing contributions from various cosmic sources and placing constraints on early black hole populations.

Contribution

It provides the first detailed power spectrum analysis of the unresolved 0.5-2 keV CXB, quantifying contributions from different source populations and constraining early black hole densities.

Findings

Unresolved CXB fluctuations account for ~12% of the total.

Galaxies, AGN, and IGM contribute 20%, 25%, and 55% respectively.

No direct evidence of Warm Hot Intergalactic Medium was found.

Abstract

In this paper we investigate the power spectrum of the unresolved 0.5-2 keV CXB with deep Chandra 4 Ms observations in the CDFS. We measured a signal which, on scales >30", is significantly higher than the Shot-Noise and is increasing with the angular scale. We interpreted this signal as the joint contribution of clustered undetected sources like AGN, Galaxies and Inter-Galactic-Medium (IGM). The power of unresolved cosmic sources fluctuations accounts for \sim 12% of the 0.5-2 keV extragalactic CXB. Overall, our modeling predicts that \sim 20% of the unresolved CXB flux is made by low luminosity AGN, \sim 25% by galaxies and \sim 55% by the IGM (Inter Galactic Medium). We do not find any direct evidence of the so called Warm Hot Intergalactic Medium (i.e. matter with 10^5K<T<10^7K and density contrast {\delta} <1000), but we estimated that it could produce about 1/7 of the unresolved CXB. We placed an upper limit to the space density of postulated X-ray-emitting early black hole at z>7.5 and compared it with SMBH evolution models.