Compton-thick accretion in the local universe

Abstract

Heavily obscured accretion is believed to represent an important stage in the growth of supermassive black holes and to play an important role in shaping the observed spectrum of the cosmic X-ray background. Hard X-ray (E > 10 keV) selected samples are less affected by absorption than samples selected at lower energies, and are therefore one of the best ways to detect and identify Compton-thick (CT, log N_H ≥ 24) active galactic nuclei (AGNs). In this letter we present the first results of the largest broadband (0.3–150 keV) X-ray spectral study of hard X-ray selected AGNs to date, focusing on the properties of heavily obscured sources. Our sample includes the 834 AGNs (728 non-blazar, average redshift z sime 0.055) reported in the 70-month catalog of the all-sky hard X-ray Swift/Burst Alert Monitor survey. We find 55 CT AGNs, which represent 7.6_-2.1^+1.1 % of our non-blazar sample. Of these, 26 are reported as candidate CT AGNs for the first time. We correct for selection bias and derive the intrinsic column density distribution of AGNs in the local universe in two different luminosity ranges. We find a significant decrease in the fraction of obscured Compton-thin AGNs for increasing luminosity, from 46 ± 3% (for log L_14-195 = 40–43.7) to 39 ± 3% (for log L_14-195 = 43.7–46). A similar trend is also found for CT AGNs. The intrinsic fraction of CT AGNs with log N_H = 24–25 normalized to unity in the N_H = 20–25 range is 27 ± 4%, and is consistent with the observed value obtained for AGNs located within 20 Mpc.

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ORCID for P. Gandhi: orcid.org/0000-0003-3105-2615

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