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

Abstract

Using Chandra observations in the 2.15 deg<SUP>2</SUP> 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<SUP>-2</SUP> s<SUP>-1</SUP> sr<SUP>-1</SUP> keV<SUP>-1</SUP>. 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<SUP>-2</SUP> s<SUP>-1</SUP> sr<SUP>-1</SUP> keV<SUP>-1</SUP> and photon spectral index Γ = 1.57 ± 0.10. Removing faint galaxies down to {I}<SUB>{AB</SUB>}∼ 27{--}28 leaves a hard spectrum with {{Γ }}∼ 1.25 and a 1 keV normalization of ∼1.37 keV cm<SUP>-2</SUP> s<SUP>-1</SUP> sr<SUP>-1</SUP> keV<SUP>-1</SUP>. 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 z∼ 6 accreted mass density, such a population of black holes must grow in Compton-thick envelopes with {N}<SUB>H</SUB> > 1.6 × 10<SUP>25</SUP> cm<SUP>-2</SUP> and form in extremely low-metallicity environments ({Z}<SUB>☉ </SUB>)∼ {10}<SUP>-3</SUP>.