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  1. OA@INAF
  2. PRODOTTI RICERCA INAF
  3. 1 CONTRIBUTI IN RIVISTE (Journal articles)
  4. 1.01 Articoli in rivista
Please use this identifier to cite or link to this item: http://hdl.handle.net/20.500.12386/26879
Title: The Chandra COSMOS Legacy Survey: Energy Spectrum of the Cosmic X-Ray Background and Constraints on Undetected Populations
Authors: CAPPELLUTI, Nico
Li, Yanxia
Ricarte, Angelo
Agarwal, Bhaskar
Allevato, Viola
Tasnim Ananna, Tonima
Ajello, Marco
Civano, Francesca
COMASTRI, Andrea 
Elvis, Martin
Finoguenov, Alexis
GILLI, Roberto 
Hasinger, Günther
MARCHESI, STEFANO 
Natarajan, Priyamvada
Pacucci, Fabio
Treister, E.
Urry, C. Megan
Issue Date: 2017
Journal: THE ASTROPHYSICAL JOURNAL 
Number: 837
Issue: 1
First Page: 19
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>.
URI: http://hdl.handle.net/20.500.12386/26879
URL: https://iopscience.iop.org/article/10.3847/1538-4357/aa5ea4
ISSN: 0004-637X
DOI: 10.3847/1538-4357/aa5ea4
Bibcode ADS: 2017ApJ...837...19C
Fulltext: open
Appears in Collections:1.01 Articoli in rivista

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