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 \rho \propto R ^ { - \lambda } , R being the radial distance from the SMBH and \lambda 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 \lambda and the disc viscosity parameter \alpha .
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 \lambda \sim 1 for viscosity parameters \alpha \sim 0.01 - 0.1 .