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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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File | Description | Size | Format | |
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Cappelluti_2017_ApJ_837_19.pdf | pdf editoriale | 769.45 kB | Adobe PDF | View/Open |
1702.01660.pdf | preprint | 343.83 kB | Adobe PDF | View/Open |
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