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

• Published in: Monthly notices of the Royal Astronomical Society Vol. 469; no. 1; pp. 314 - 322
• Main Authors: Kathirgamaraju, Adithan, Barniol Duran, Rodolfo, Giannios, Dimitrios
• Format: Journal Article
• Language: English
• Published: Oxford University Press 01.07.2017
• Subjects: methods: analytical; galaxies: nuclei; accretion, accretion discs
• ISSN: 0035-8711; 1365-2966
• DOI: 10.1093/mnras/stx846

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 ρ ∝ R−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.