On the Fraction of X-Ray-obscured Quasars in the Local Universe

Recent wide-area hard X-ray and soft gamma-ray surveys have shown that the fraction of X-ray-obscured active galactic nuclei (AGNs) in the local universe significantly decreases with intrinsic luminosity. In this Letter we point out that two corrections have to be made to the samples: (1) radio-loud AGNs have to be excluded, since their X-ray emission might be dominated by the jet component, and (2) Compton-thick sources have to be excluded too, since their hard X-ray and soft gamma-ray emission are also strongly attenuated by Compton scattering. The soft gamma-ray-selected AGN samples obtained by Swift and INTEGRAL provide the best opportunity to study the fraction of obscured AGNs in the local universe in the least biased way. We choose these samples to check if the corrections could alter the above result on the fraction of obscured AGNs. We find that before the corrections both samples show significant anticorrelation between LX and NH, indicating an obvious decrease in the fraction of obscured AGNs with luminosity. However, after the corrections, we find only marginal evidence of anticorrelation (at the 98% confidence level) in the Swift sample and no evidence at all in the INTEGRAL sample, which consists of a comparable number of objects. We conclude that current samples only show a marginal decrease in the fraction of obscured AGNs in the local universe and that much larger samples are required in order to reach a more robust conclusion.