The Chandra COSMOS Legacy Survey: Energy Spectrum of the Cosmic X-Ray Background and Constraints on Undetected Populations

Using Chandra observations in the 2.15 deg² 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 ∼ 0.27 and 0.07 keV and an extragalactic power law with a photon spectral index Γ = 1.45 - 0.02. The 1 keV normalization of the extragalactic component is 10.91 - 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 - 0.26 keV cm^-2 s^-1 sr^-1 keV^-1 and photon spectral index Γ = 1.57 - 0.10. Removing faint galaxies down to leaves a hard spectrum with and a 1 keV normalization of ∼1.37 keV cm^-2 s^-1 sr^-1 keV^-1. This means that ∼91% of the observed CXB is resolved into detected X-ray sources and undetected galaxies. Unresolved sources that contribute ∼8%-9% of the total CXB show marginal evidence of being harder and possibly more obscured than resolved sources. Another ∼1% of the CXB can be attributed to still undetected star-forming galaxies and absorbed active galactic nuclei. 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 accreted mass density, such a population of black holes must grow in Compton-thick envelopes with 1.6 - 10²⁵ cm^-2 and form in extremely low-metallicity environments.