A model for the X-ray absorption in Compton-thin AGNA. Lamastra, G. C. Perola and G. Matt
Dipartimento di Fisica "E. Amaldi", Università degli Studi Roma Tre, via della Vasca Navale 84, 00146 Roma, Italy
(Received 9 August 2005 / Accepted 30 November 2005 )
The fraction of AGN with photoelectric absorption in the X-rays ranging from of 1022 up to about 1024 cm-2 (Compton-thin) appears observationally to be anticorrelated to their luminosity Lx. This recently found evidence is used to investigate the location of the absorbing gas. The molecular torus invoked in the unified picture of AGN, while it can be regarded as confirmed on several grounds to explain the Compton-thick objects, do not conform to this new constraint, at least in its physical models as developed so far. In the frame of observationally based evidence that in Compton-thin sources the absorbing gas might be located far away from the X-ray source, it is shown that the gravitational effects of the black hole (BH) on the molecular gas in a disk, within 25-450 pc (depending on the BH mass, from 106 to 10 ), leads naturally to the observed anticorrelation, under the assumption of a statistical correlation between the BH mass and Lx. Its normalization is also reproduced provided that the surface density, , of this gas is larger than about 150-200 pc-2, and assuming that the bolometric luminosity is one tenth of the Eddington limit. Interestingly, the required values are consistent with the value of the 300 pc molecular disk in our own galaxy, namely 500 pc-2. In a sample of nearby galaxies from the BIMA SONG survey, it is found that half of the objects have central larger than 150 pc-2. Given the simplicity of the proposed model, this finding is very encouraging, waiting for future higher resolution surveys in CO on more distant galaxies.
Key words: galaxies: active -- X-rays: galaxies -- ISM: clouds
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