The synthesis of the cosmic X-ray background in the Chandra and XMM-Newton era

By R. Gilli, A. Comastri, G. Hasinger. 2006

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We present a detailed and self-consistent modeling of the cosmic X-ray background (XRB) based on the most up-to-date X-ray luminosity functions (XLF) and evolution of Active Galactic Nuclei (AGN). The large body of observational results collected by soft (0.5–2 keV) and hard (2–10 keV) X-ray surveys are used to constrain at best the properties of the Compton-thin AGN population and its contribution to the XRB emission. The number ratio R between moderately obscured (Compton-thin) AGN and unobscured AGN is fixed by the comparison between the soft and hard XLFs, which suggests that R decreases from 4 at low luminosities to 1 at high luminosities. From the same comparison there is no clear evidence of an evolution of the obscured AGN fraction with redshift. The distribution of the absorbing column densities in obscured AGN is determined by matching the soft and hard source counts. A distribution rising towards larger column densities is able to reproduce the soft and hard AGN counts over about 6 dex in flux. The model also reproduces with excellent accuracy the fraction of obscured objects in AGN samples selected at different X-ray fluxes. The integrated emission of the Compton-thin AGN population is found to underestimate the XRB flux at about 30 keV, calling for an additional population of extremely obscured (Compton-thick) AGN. Since the number of Compton-thick sources required to fit the 30 keV XRB emission strongly depends on the spectral templates assumed for unobscured and moderately obscured AGN, we explored the effects of varying the spectral templates. In particular, in addition to the column density distribution, we also considered a distribution in the intrinsic powerlaw spectral indices of variable width. In our baseline model a Gaussian distribution of photon indices with mean $\langle \Gamma \rangle=1.9$ and dispersion $\sigma_{\Gamma}=0.2$ is assumed. This increases the contribution of the Compton-thin AGN population to the 30 keV XRB intensity by ~$30\%$ with respect to the case of null dispersion (i.e. a single primary AGN powerlaw with $\Gamma=1.9$) but is not sufficient to match the 30 keV XRB emission. Therefore a population of heavily obscured -Compton-thick- AGN, as large as that of moderately obscured AGN, is required to fit the residual background emission. Remarkably, the fractions of Compton-thick AGN observed in the Chandra Deep Field South and in the first INTEGRAL and Swift catalogs of AGN selected above 10 keV are in excellent agreement with the model predictions.

Subfields: Astronomy and Astrophysics, Nuclear and High Energy Physics

View Paper: https://www.semanticscholar.org/paper/d35d217b41732311b36951ae7edd08bfbc20c916

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Citations in the corpus

THE SPACE DENSITY OF COMPTON-THICK ACTIVE GALACTIC NUCLEUS AND THE X-RAY BACKGROUND (2009, methodology): “At fluxes of ∼10−11 erg cm−2 −1 the fraction of CT AGN in the model of Gilli et al. (2007) is ∼15%, while the observed value is ∼6±5% for INTEGRAL, and ∼8±3% for the Swift sample.”
Active galactic nuclei: what’s in a name? (2017, background): “3.1), particularly as a function of luminosity (e.g. Gilli et al. 2007; Ueda et al. 2014).”
Swift BAT Survey of AGNs (2007, methodology): “Considering that the bulk of the objects and their emitted luminosity lie near the break luminosity, this could imply a substantial modification to the present-day evolution models (e.g., Gilli et al. 2007).”
Suzaku Observations of Active Galactic Nuclei Detected in the Swift BAT Survey: Discovery of a “New Type” of Buried Supermassive Black Holes (2007, methodology): “According to synthesis models of the X-ray background (XRB) (e.g., Ueda et al. 2003; Gilli et al. 2007), much of the peak intensity of the XRB at 30 keV should be produced by these objects.”
THE XMM-NEWTON WIDE-FIELD SURVEY IN THE COSMOS FIELD (XMM-COSMOS): DEMOGRAPHY AND MULTIWAVELENGTH PROPERTIES OF OBSCURED AND UNOBSCURED LUMINOUS ACTIVE GALACTIC NUCLEI (2010, methodology): “At LX ∼> 10 44 erg s−1, the obscured AGN fraction is of the order of 25-30% in the logLX = 44 − 44.5 bin and ∼ 15 − 20% in the logLX = 44.5 − 45 bin, about a factor of 2-3 lower than the predictions from the Gilli et al. (2007) and Treister & Urry (2006) models.”
IDENTIFYING LUMINOUS ACTIVE GALACTIC NUCLEI IN DEEP SURVEYS: REVISED IRAC SELECTION CRITERIA (2012, background): “Because high luminosity AGN tend to be less obscured than their low luminosity counterparts, at least in X-ray selected samples, we might expect sources with redder IRAC colors to have a lower Type 2 fraction (e.g. Ueda et al. 2003; Gilli et al. 2007; Hasinger 2008).”
Faint high-redshift AGN in the Chandra deep field south: the evolution of the AGN luminosity function and black hole demography (2011, methodology): “This is higher than predicted by popular models for the CXB (Gilli et al. 2007).”
THE CHANDRA COSMOS SURVEY. I. OVERVIEW AND POINT SOURCE CATALOG (2009, positive): “5 following the predictions of Gilli et al. (2007) XRB model6.”
Tracing the cosmic growth of supermassive black holes to z ∼ 3 with Herschel (2014, background): “MH08 estimated the fraction of highlyobscured (NH > 1.5×1024 cm−2) AGN from the X-ray background (XRB) sythesis models by Gilli et al. (2007).”
Unveiling Obscured Accretion in the Chandra Deep Field-South (2007, methodology): “We find 46±10 24µm selected, highly obscured AGNs, a number about twice the La Franca et al. (2005) prediction but well consistent with the Gilli et al. (2007) prediction.”
BAT AGN Spectroscopic Survey. V. X-Ray Properties of the Swift/BAT 70-month AGN Catalog (2017, background): “…is emitted, suggests that heavily obscured AGN contribute significantly to the CXB. Synthesis models of the CXB (e.g., Ueda et al. 2003, 2014, Gilli et al. 2007, Treister & Urry 2005; Treister et al. 2009, Draper & Ballantyne 2010, Akylas et al. 2012) have shown that a fraction of 10−30%…”
He ii REIONIZATION AND ITS EFFECT ON THE INTERGALACTIC MEDIUM (2008, methodology): “Models of the obscured contribution to the diffuse X-ray background find that obscured quasars (defined as having QSOs with obscuring columns of NHI > 10 21 cm−2) begin to dominate the background above approximately 2 keV (e.g., Gilli et al. 2007).”
X-RAY SPECTRAL PROPERTIES OF THE BAT AGN SAMPLE (2008, background): “The most recent synthesis models for the cosmic Xray background (CXB) find that heavily obscured and Compton thick AGN are as important as unabsorbed AGN (Gilli et al. 2007).”
Dust covering factor, silicate emission, and star formation in luminous QSOs (2007, background): “La Franca et al. (2005) and Akylas et al. (2006) find evidence for an increasing fractionof obscured AGNs as a function of redshift, a result which is still debated (see Ueda et al., 2003; Gilli et al., 2007).”
THREE-YEAR SWIFT–BAT SURVEY OF ACTIVE GALACTIC NUCLEI: RECONCILING THEORY AND OBSERVATIONS? (2010, methodology): “In addition, our findings agree with the modeling of obscured sources in Gilli et al. (2007), where the reflection efficiency for high inclination angles (expected for obscured AGNs in the unified picture) is lower (0.88 rather than 1.3) than the one assumed for unobscured ones.”
COMPTON-THICK ACCRETION IN THE LOCAL UNIVERSE (2015, methodology): “In agreement with the results of Tueller et al. (2008), we find that the models of Draper & Ballantyne (2010) and Gilli et al. (2007) clearly overestimate (by a factor of ∼2) the fraction of CT AGNs for the flux limit we probe in the 10–40 keV band.”
Cosmic X-Ray Background and Earth Albedo Spectra with Swift BAT (2008, background): “Almost all of the CXB radiation up to 300 keV can be explained in terms of emission-line AGNs (e.g., Gilli et al. 2007).”