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

Journal Article · Wed Mar 01 04:00:00 EST 2017 · Astrophysical Journal

DOI:https://doi.org/10.3847/1538-4357/AA5EA4· OSTI ID:22869290

Cappelluti, Nico; Li, Yanxia; Agarwal, Bhaskar; Ananna, Tonima Tasnim; Natarajan, Priyamvada; Urry, C. Megan ^[1]; Ricarte, Angelo ^[2]; Allevato, Viola; Finoguenov, Alexis ^[3]; Ajello, Marco; Marchesi, Stefano ^[4]; Civano, Francesca; Elvis, Martin ^[5]; Comastri, Andrea; Gilli, Roberto ^[6]; Hasinger, Günther ^[7]; Pacucci, Fabio ^[8]; Treister, E. ^[9]

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1. Yale Center for Astronomy and Astrophysics, P.O. Box 208121, New Haven, CT 06520 (United States)
2. Department of Astronomy, Yale University, P.O. Box 208101, New Haven, CT 06520 (United States)
3. Department of Physics, University of Helsinki, Gustaf Hällströmin katu 2a, FI-00014 Helsinki (Finland)
4. Department of Physics and Astronomy, Clemson University, Kinard Lab of Physics, Clemson, SC 29634-0978 (United States)
5. Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138 (United States)
6. INAF-Osservatorio Astronomico di Bologna, via Ranzani 1, I-40127 Bologna (Italy)
7. Institute for Astronomy, 2680 Woodlawn Drive, University of Hawaii, Honolulu, HI 96822 (United States)
8. Department of Physics, Yale University, P.O. Box 208121, New Haven, CT 06520 (United States)
9. Instituto de Astrofisica, Facultad de Fisica, Pontificia Universidad Catolica de Chile, Casilla 306, Santiago 22 (Chile)

Using Chandra observations in the 2.15 deg{sup 2} 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} s{sup −1} sr{sup −1} keV{sup −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{sup −2} s{sup −1} sr{sup −1} keV{sup −1} and photon spectral index Γ = 1.57 ± 0.10. Removing faint galaxies down to i{sub AB}∼27--28 leaves a hard spectrum with Γ∼1.25 and a 1 keV normalization of ∼1.37 keV cm{sup −2} s{sup −1} sr{sup −1} keV{sup −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{sub H} > 1.6 × 10{sup 25} cm{sup −2} and form in extremely low-metallicity environments (Z{sub ⊙})∼10{sup −3}.

• OSTI ID: 22869290
• Journal Information: Astrophysical Journal, Journal Name: Astrophysical Journal Journal Issue: 1 Vol. 837; ISSN ASJOAB; ISSN 0004-637X
• Country of Publication: United States
• Language: English

Related Subjects

79 ASTRONOMY AND ASTROPHYSICS
BLACK HOLES
CATALOGS
COSMIC PHOTONS
DENSITY
ENERGY SPECTRA
GALAXIES
GALAXY NUCLEI
KEV RANGE
MASS
METALLICITY
PLASMA
QUASARS
STARS
UNIVERSE
X RADIATION
X-RAY SOURCES