"Comets" orbiting a black hole
R. Maiolino, G. Risaliti, M. Salvati, P. Pietrini, G., Torricelli-Ciamponi, M. Elvis, G. Fabbiano, V. Braito, J. Reeves

TL;DR
This study uses X-ray observations of NGC1365 to reveal that the broad-line region clouds have a cometary shape with elongated tails, suggesting a dynamic, transient nature influenced by hydrodynamical effects.
Contribution
It provides the first observational evidence that BLR clouds are elongated and cometary, challenging the traditional spherical cloud models and proposing a new shape influenced by hydrodynamical processes.
Findings
BLR clouds have elongated, cometary shapes with dense heads and expanding tails.
The tails are longer than a few times 10^13 cm and have small opening angles.
BLR clouds likely have short lifetimes of a few months, requiring continuous replenishment.
Abstract
We use a long (300 ksec), continuous Suzaku X-ray observation of the active nucleus in NGC1365 to investigate the structure of the circumnuclear 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=10^11 cm^-3) and an expanding, dissolving tail. We infer that the cometary tails must be longer than a few times 10^13 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…
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