“Comets” orbiting a black hole

¹ INAF-Osservatorio Astronomico di Roma, via di Frascati 33, 00040 Monte Porzio Catone, Italy e-mail: maiolino@oa-roma.inaf.it
² Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy
³ Harvard-Smithsonian Center for Astrophysics, 60 Garden St. Cambridge, MA 02138, USA
⁴ Dipartimento di Astronomia, Università di Firenze, largo E. Fermi 2, 50125 Firenze, Italy
⁵ Department of Physics and Astronomy, University of Leicester, University Road, Leicester, LE1 7RH, UK
⁶ Department of Physics and Astronomy, Johns Hopkins University, Baltimore, MD 21218, USA
⁷ Astrophysics Group, School of Physical and Geographical Science, Keele University, Keele, Staffordshire ST5 5BG, UK

Received: 30 December 2009
Accepted: 19 April 2010

Abstract

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.