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

Nico Cappelluti, Yanxia Li, Angelo Ricarte, Bhaskar Agarwal, Viola Allevato, Tonima Tasnim Ananna, Marco Ajello, Francesca Civano, Andrea Comastri, Martin Elvis, Alexis Finoguenov, Roberto Gilli, Günther Hasinger, Stefano Marchesi, Priyamvada Natarajan, Fabio Pacucci, E. Treister, C. Megan Urry

Research output: Contribution to journalArticlepeer-review

49 Scopus citations


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 leaves a hard spectrum with 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 accreted mass density, such a population of black holes must grow in Compton-thick envelopes with 1.6 - 1025 cm-2 and form in extremely low-metallicity environments.

Original languageEnglish (US)
Article number19
JournalAstrophysical Journal
Issue number1
StatePublished - Mar 1 2017
Externally publishedYes


  • X-rays: diffuse background
  • catalogs
  • infrared: diffuse background
  • quasars: supermassive black holes
  • surveys

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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