Rong-Feng Shen, Christopher D. Matzner
In a stellar tidal disruption event (TDE), an accretion disk forms as the stellar debris returns and circularizes. Rather than being confined within the circularizing radius, the disk can spread to larger radii to conserve angular momentum. An outer spreading disk is a source of matter for re-accretion at rates which can exceed the later stellar fall-back rate, although a disk wind can suppress its contribution to the central black hole accretion rate. A spreading disk is detectible through a break in the central accretion rate history, or, at longer wavelengths, by its own emission. Moreover, as an angular momentum reservoir, it can broadcast its existence by affecting the disk precession rate. Because these features depend on the disk's internal viscosity and the nature of wind produced in its early, advection-dominated phase, they are useful probes of transient disk physics. To model the evolution of TDE disk size and accretion rate, we account for the possibility of thermal instability for accretion rates intermediate between the advection-dominated and gas-pressure dominated states, for the influence of wind losses in the advective stage, and for the case in which the disk precesses away from its original plane due to Lense-Thirring effect. All or part of a young TDE disk will precess as a solid body and precession may manifest itself as quasi-periodic modulation of light curve. The disk precession period increases with time at a rate depending on disk evolution. Applying our results to the jetted TDE candidate Sw J1644+57, whose X-ray light curve shows numerous quasi-periodic dips, we argue that the data best fit a scenario in which the star plunged deeply within its tidal radius on an orbit significantly inclined from the black hole equator, with the apparent jet shutoff at t= 500 d corresponding to a disk transition from the advective state to the gas-pressure dominated state.
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http://arxiv.org/abs/1305.5570
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