科研之友
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摘要
We present an analysis of the deepest hard X-ray survey to date of about 2500 deg(2) performed by the IBIS instrument on board INTEGRAL in the 20-60 keV band, with a total exposure time of 4 Ms. We find 34 candidate sources, for which we try to find counterparts at other wavelengths. The ratio of Seyfert 1 to Seyfert 2 is significantly more than the ratio found in the optical. This effect may be explained in the framework of the receding-torus model, but could also be due to absorption columns large enough to affect the 20-60 keV band. None of the predicted Compton-thick objects with 10(24) < N(H) < 10(25) cm(-2) is detected unambiguously; when taking lower limits on N(H) into account, the fraction of these objects is found to be lower than 24%. We do not see, but cannot exclude, a relationship between absorption and luminosity similar to what is seen in the 2-10 keV band. Our data suggests the possibility of a lack of objects with 10(21) <= NH <= 10(22) cm(-2), which could be expected if absorption has two origins, for instance a torus-like structure and the host galaxy. We find that the Log N-Log S diagram of our sources is compatible with those obtained in other surveys in hard X-rays. Compared to models of the AGN population selected in the 2-10 keV band, the Log N-Log S diagram is generally in good agreement, but the NH distribution is significantly different, with significantly less unabsorbed sources (N(H) < 10(22) cm(-2)) at a given flux limit compared to the models. In this survey, we resolve about 2.5% of the cosmic X-ray background in the 20-60 keV band. We also study the local hard X-ray luminosity function, which is compatible with what is found in other recent hard X-ray surveys. The characteristic luminosity Log L(20-60 keV)* = 43.66 is found to be a factor about 5 lower than the value observed in the 2-10 keV band. We find a space density of 10(-3) AGN with L(20-60 keV) > 10(41) per Mpc(3) and a corresponding luminosity density of 0.9 x 10(39) erg s(-1) Mpc(-3).
- 出版日期2008-7
