Maiolino, R., Risaliti, Guido, Salvati, M., Pietrini, P., Torricelli-Ciamponi, G., Elvis, Martin, Fabbiano, Giuseppina, Braito, V., and Reeves, J. 2010. ""Comets" orbiting a black hole." Astronomy and Astrophysics, 517 47.
We use a long (300 ks), continuous Suzaku X-ray observation of the active nucleus in NGC 1365 to investigate the structure of the circumnuclear broad line region (BLR) clouds through their occultation of the X-ray source. The variations of the absorbing column density and of the covering factor indicate that the clouds surrounding the black hole are far from having a spherical geometry (as sometimes assumed), instead they have a strongly elongated and cometary shape, with a dense head (n ~ 1011 cm-3) and an expanding, dissolving tail. We infer that the cometary tails must be longer than a few times 1013 cm and their opening angle must be smaller than a few degrees. We suggest that the cometary shape may be a common feature of BLR clouds in general, but which has been difficult to recognize observationally so far. The cometary shape may originate from shocks and hydrodynamical instabilities generated by the supersonic motion of the BLR clouds into the intracloud medium. As a consequence of the mass loss into their tail, we infer that the BLR clouds probably have a lifetime of only a few months, implying that they must be continuously replenished. We also find a large, puzzling discrepancy (two orders of magnitude) between the mass of the BLR inferred from the properties of the absorbing clouds and the mass of the BLR inferred from photoionization models; we discuss the possible solutions to this discrepancy.
