The Chandra COSMOS legacy survey: Energy Spectrum of the Cosmic X-ray
  Background and constraints on undetected populations
  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 $\Gamma$=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
$\Gamma$=1.57$\pm{0.10}$. Removing faint galaxies down to i$_{AB}\sim$27-28
leaves a hard spectrum with $\Gamma\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 $(Z_\odot)\sim10^{-3}$.
