On the fraction of X-ray obscured quasars in the local universe

Abstract

Recent wide area hard X-ray and soft Gamma ray surveys have shown that the fraction of X-ray obscured AGNs in the local universe significantly decreases with intrinsic luminosity. In this letter we point out that two correction 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; 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 AGN 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 anti-correlation between L_X and N_H, indicating obvious decrease in the fraction of obscured AGNs with luminosity. However, after the corrections, we find only marginal evidence of anti-correlation (at 98% confidence level) in the SWIFT sample, and no evidence at all in the INTEGRAL sample which consists of 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 much larger samples are required to reach a more robust conclusion.

13 pages, including 4 figures, ApJ letter accepted

References in corpus (4)

• The Serendipitous Extragalactic X-Ray Source Identification (SEXSI) Program. III. Optical Spectroscopy
• On the X-ray Baldwin effect for narrow Fe Kalpha emission line
• XMM-Newton observations of a sample of gamma-ray loud active galactic nuclei
• Spectral Statistics and Local Luminosity Function of a Hard X-ray Complete Sample of Brightest AGNs

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• Constraints on light bending and reflection from the hard X-ray background
• Most hard X-ray selected quasars in Chandra Deep Fields are obscured
• Rapidly spinning massive black holes in active galactic nuclei: evidence from the black hole mass function