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

Journal
Date Issued
2017
Author(s)
CAPPELLUTI, Nico
•
Li, Yanxia
•
Ricarte, Angelo
•
Agarwal, Bhaskar
•
•
Tasnim Ananna, Tonima
•
Ajello, Marco
•
Civano, Francesca
•
•
Elvis, Martin
•
Finoguenov, Alexis
•
•
Hasinger, Günther
•
•
Natarajan, Priyamvada
•
Pacucci, Fabio
•
Treister, E.
•
Urry, C. Megan
DOI
Abstract
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 {I}{AB}∼ 27{--}28 leaves a hard spectrum with {{Γ }}∼ 1.25 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 z∼ 6 accreted mass density, such a population of black holes must grow in Compton-thick envelopes with {N}H > 1.6 × 1025 cm-2 and form in extremely low-metallicity environments ({Z}☉ )∼ {10}-3.
Volume
837
Issue
1
Start page
19
Uri
Url
Issn Identifier
0004-637X
Ads BibCode
2017ApJ...837...19C
Rights
open.access
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