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 â¼ 0.27 and 0.07 keV and an extragalactic power law with a photon spectral index Î = 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 Î = 1.57 - 0.10. Removing faint galaxies down to leaves a hard spectrum with and a 1 keV normalization of â¼1.37 keV cm-2 s-1 sr-1 keV-1. This means that â¼91% of the observed CXB is resolved into detected X-ray sources and undetected galaxies. Unresolved sources that contribute â¼8%-9% of the total CXB show marginal evidence of being harder and possibly more obscured than resolved sources. Another â¼1% 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.
|Titolo:||The Chandra COSMOS Legacy Survey: Energy Spectrum of the Cosmic X-Ray Background and Constraints on Undetected Populations|
|Data di pubblicazione:||2017|
|Digital Object Identifier (DOI):||http://dx.doi.org/10.3847/1538-4357/aa5ea4|
|Parole Chiave:||catalogs; infrared: diffuse background; quasars: supermassive black holes; surveys; X-rays: diffuse background; Astronomy and Astrophysics; Space and Planetary Science|
|Appare nelle tipologie:||1.1 Articolo in rivista|