Probing the Cosmic X-ray and MeV Gamma-ray Background Radiation through the Anisotropy Page: 3 of 48
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17 1. Introduction
18 The cosmic X-ray background (CXB) is an isotropic, apparently diffuse X-ray emission in
19 the Universe which was discovered about 50 years ago (Giacconi et al. 1962). It is often assumed
20 that the CXB has been conclusively shown to be the integrated light produced via the accretion
21 process of active galactic nuclei (AGNs), in particular Seyferts, hosting supermassive black holes.
22 This might be correct below - 5 keV. Emission from active galaxies has indeed been resolved by
23 the deep X-ray surveys by Chandra in the broad 0.5-2 keV and 2-10 keV bands. Those objects
24 account for 80-90% of the CXB (Mushotzky et al. 2000; Giacconi et al. 2002; Alexander et al.
25 2003a,b; Bauer et al. 2004). However, energy-resolved studies indicate that the resolved fraction
26 of the CXB decreases with energy as 80-90% over 2-8 keV, ~ 60% over 6-8 keV, and ~ 50%
27 beyond 8 keV (Worsley et al. 2004, 2005).
28 Above - 2 keV, the CXB cannot be due to superposition of unabsorbed AGNs, mainly
29 type I Seyferts. Those objects show a typical continuum photon index of F = 1.9 below 10 keV
30 (Nandra & Pounds 1994; Reeves & Turner 2000; Piconcelli et al. 2005), different from that of
31 CXB F = 1.4 at 2-8 keV (De Luca & Molendi 2004). Instead, this unresolved, hard component
32 is generally attributed to the emission from absorbed Seyferts, the so-called type II Seyferts,
33 which might be buried in dusty tori. A superposition of such sources with varying degrees of
34 photoelectric absorption by the circumnuclear material can cause the total spectrum to appear
35 harder than spectra of unabsorbed Seyferts, but this requires some fine-tuning of absorption
36 properties of sources as a function of redshift and luminosity. Various population synthesis models
37 successfully explain the CXB by introducing appropriate number of absorbed Seyferts (see e.g.
38 Ueda et al. 2003; Treister & Urry 2005; Gilli et al. 2007). However, recent studies (Treister et al.
39 2009) showed that the number of Compton thick AGNs, which are a class of absorbed Seyferts
40 and whose column density is larger than the inverse of the Thomson cross section, is a factor of
41 3-4 less than that expected in the population synthesis models at least locally (see also Ajello
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Inoue, Yoshiyuki; Murase, Kohta; Madejski, Grzegorz M. & Uchiyama, Yasunobu. Probing the Cosmic X-ray and MeV Gamma-ray Background Radiation through the Anisotropy, article, August 15, 2013; United States. (https://digital.library.unt.edu/ark:/67531/metadc829803/m1/3/: accessed April 25, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.