The Chandra COSMOS legacy survey: Energy Spectrum of the Cosmic X-ray Background and constraints on undetected populations

Abstract

Using {\em Chandra} observations in the 2.15 deg${}^2$ COSMOS legacy field, we present one of the most accurate measurements of the Cosmic X-ray Background (CXB) spectrum to date in the [0.3-7] keV energy band. The CXB has three distinct components: contributions from two Galactic collisional thermal plasmas at kT$\sim$0.27 and 0.07 keV and an extragalactic power-law with photon spectral index $\hat{I}“$=1.45$\pm 0.02$. The 1 keV normalization of the extragalactic component is 10.91$\pm 0.16$ keV cm${}^{-2}$ s${}^{-1}$ sr${}^{-1}$ keV${}^{-1}$. Removing all X-ray detected sources, the remaining unresolved CXB is best-fit by a power-law with normalization 4.18$\pm 0.26$ keV cm${}^{-2}$ s${}^{-1}$ sr${}^{-1}$ keV${}^{-1}$ and photon spectral index $\hat{I}“$=1.57$\pm 0.10$. Removing faint galaxies down to i${}_{AB}\sim$27-28 leaves a hard spectrum with $\hat{I}“\sim$1.25 and a 1 keV normalization of $\sim$1.37 keV cm${}^{-2}$ s${}^{-1}$ sr${}^{-1}$ keV${}^{-1}$. This means that $\sim$91\% of the observed CXB is resolved into detected X-ray sources and undetected galaxies. Unresolved sources that contribute $\sim 8-9\%$ of the total CXB show a marginal evidence of being harder and possibly more obscured than resolved sources. Another $\sim$1\% of the CXB can be attributed to still undetected star forming galaxies and absorbed AGN. According to these limits, we investigate a scenario where early black holes totally account for non source CXB fraction and constrain some of their properties. In order to not exceed the remaining CXB and the $z\sim$6 accreted mass density, such a population of black holes must grow in Compton-thick envelopes with N${}_H>$1.6$\times$10${}^{25}$ cm${}^{-2}$ and form in extremely low metallicity environments $(Z_{\odot })\sim 10^{-3}$.

10 Pages, 6 Figures, Accepted by ApJ