TDE fallback cut-off due to a pre-existing accretion disc

Abstract

Numerous tidal disruption event (TDE) candidates originating from galactic centres have been detected (e.g. by Swift and ASASSN). Some of their host galaxies show typical characteristics of a weak active galactic nucleus (AGN), indicative of a pre-existing accretion disc around the supermassive black hole (SMBH). In this work, we develop an analytic model to study how a pre-existing accretion disc affects a TDE. We assume the density of the disc ραR-λ, R being the radial distance from the SMBH and λ varying between 0.5 and 1.5. Interactions between the pre-existing accretion disc and the stream of the tidally disrupted star can stall the stream far from the SMBH, causing a sudden drop in the rate of fallback of gas into the SMBH. These interactions could explain the steep cut-off observed in the light curve of some TDE candidates (e.g. Swift J1644 and Swift J2058). With our model, it is possible to use the time of this cut-off to constrain some properties pertaining to the pre-existing accretion disc, such as λ and the disc viscosity parameter α. We demonstrate this by applying our theory to the TDE candidates Swift J1644, Swift J2058 and ASASSN-14li. Our analysis favours a disc profile with λ ∼ 1 for viscosity parameters α ∼ 0.01-0.1.