Compton-thick Accretion in the Local Universe

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 (CXB). Hard X-ray (E>10keV) 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, logNH ≥ 24) Active Galactic Nuclei (AGN). In this letter we present the first results of the largest broad-band (0.3–150keV) X-ray spectral study of hard X-ray selected AGN to date, focusing on the properties of heavily obscured sources. Our sample includes the 834 AGN (728 non-blazar, average redshift z ≃ 0.055) reported in the 70-months catalog of the all-sky hard X-ray Swift/BAT survey. We find 55 CT AGN, which represent 7.6 −2.1% of our non-blazar sample. Of these, 26 are reported as candidate CT AGN for the first time. We correct for selection bias and derive the intrinsic column density distribution of AGN in the local Universe in two different luminosity ranges. We find a significant decrease in the fraction of obscured Compton-thin AGN for increasing luminosity, from 46± 3% (for logL 14−195 = 40− 43.7) to 39± 3% (for logL 14−195 = 43.7− 46). A similar trend is also found for CT AGN. The intrinsic fraction of CT AGN with logNH = 24 − 25 normalised to unity in the logNH = 20 − 25 range is 27 ± 4%, and is consistent with the observed value obtained for AGN located within 20Mpc.