Compton-thick AGN in the 70-monthSwift-BAT All-Sky Hard X-ray Survey: A Bayesian approach

Abstract

The 70-month Swift/BAT catalogue provides a sensitive view of the extragalactic X-ray sky at hard energies (>10 keV) containing about 800 active galactic nuclei (AGN). We explore its content in heavily obscured, Compton-thick AGN by combining the BAT (14-195 keV) with the lower energy XRT (0.3-10 keV) data. We apply a Bayesian methodology using Markov chains to estimate the exact probability distribution of the column density for each source. We find 53 possible Compton-thick sources (probability range 3 -100%) translating to a ∼7% fraction of the AGN in our sample. We derive the first parametric luminosity function of Compton-thick AGN. The unabsorbed luminosity function can be represented by a double power law with a break at L ∼ 2 × 10 42 ergs s −1 in the 20-40 keV band. The Compton-thick AGN contribute ∼17% of the total AGN emissivity. We derive an accurate Compton-thick number count distribution taking into account the exact probability of a source being Compton-thick and the flux uncertainties. This number count distribution is critical for the calibration of the X-ray background synthesis models, i.e. for constraining the intrinsic fraction of Compton-thick AGN. We find that the number counts distribution in the 14-195 keV band agrees well with our models which adopt a low intrinsic fraction of Compton-thick AGN (∼ 12%) among the total AGN population and a reflected emission of ∼ 5%. In the extreme case of zero reflection, the number counts can be modelled with a fraction of at most 30% Compton-thick AGN of the total AGN population and no reflection. Moreover, we compare our X-ray background synthesis models with the number counts in the softer 2-10 keV band. This band is more sensitive to the reflected component and thus helps us to break the degeneracy between the fraction of Compton-thick AGN and the reflection emission. The number counts in the 2-10 keV band are well above the models which assume a 30% Compton-thick AGN fraction and zero reflection, while they are in better agreement with models assuming 12% Compton-thick fraction and 5% reflection. The only viable alternative for models invoking a high number of Compton-thick AGN is to assume evolution in their number with redshift. For example, in the zero reflection model the intrinsic fraction of Compton-thick AGN should rise from 30% at redshift z∼ 0 to about 50% at a redshift of z=1.1.

X-ray constraints on the fraction of obscured active galactic nuclei at high accretion luminosities

“…Also shown are the recent constraints from Akylas et al (2016) on the luminosity function of Compton-thick AGNs using the 70-month Swift-BAT all-sky survey hard X-ray catalogue. The Akylas et al (2016) space densities are lower than previous studies, including those from Ajello et al (2012). This discrepancy is surprising, given that the two studies find very similar observed fractions of Compton-thick sources among the total AGN sample.…”

“…The number of Compton-thick sources among AGNs is still debated with different studies finding discrepant results in terms of both space density and cosmological evolution (e.g. Brightman & Ueda 2012;Ueda et al 2014;Ricci et al 2015;Buchner et al 2015;Akylas et al 2016).…”

Black Hole Growth Is Mainly Linked to Host-galaxy Stellar Mass Rather Than Star Formation Rate

“…The prevalence of CTK AGNs is predicted by population-synthesis models of the cosmic X-ray background, although these models have significant uncertainties in the regime of very high N H (e.g., Gilli et al 2007;Ueda et al 2014). The predicted total X-ray emission from CTK AGNs is usually less than that from other AGNs (e.g., Gilli et al 2007;Treister et al 2009; also see Buchner et al 2015, for relevant spectral fitting), consistent with the very hard X-ray (10 keV) observations of the local universe (e.g., Akylas et al 2016;Koss et al 2016). Even if these CTK AGNs predominantly reside in low-M * systems, the total L X from these galaxies will be at most the same as that from high-M * galaxies; the average L X in low-M * galaxies should be still much lower than for massive galaxies, as the former has much larger number density.…”

Suzaku Observations of Heavily Obscured (Compton-thick) Active Galactic Nuclei Selected by the Swift/BAT Hard X-Ray Survey

“…From the Swift/BAT 70-month catalog (Baumgartner et al 2013), Ricci et al (submitted to ApJS) systematically analyze X-ray data below 10 keV obtained mainly with Swift/X-Ray Telescope (XRT) and XMM-Newton, which are combined with the Swift/BAT spectra in the 14-150 keV. As a result, Ricci et al (2015) identify 55 CTAGN candidates (see also Akylas et al 2016). However, since the photon statistics of the Swift/XRT spectrum is limited and the data above and below 10 keV were not simultaneously obtained (thus being affected by time variability), large uncertainties remain in the derived column density and intrinsic luminosity.…”