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

## TL;DR

This paper presents an analytic model showing how a pre-existing accretion disc can cause a sharp cutoff in TDE light curves, enabling constraints on disc properties from observed TDEs.

## Contribution

The study introduces a new analytic framework linking TDE fallback cut-offs to properties of pre-existing accretion discs, with application to real TDE candidates.

## Key findings

- Disc interactions can cause fallback rate drops in TDEs.
- Model constrains disc density profile and viscosity from light curve cut-offs.
- Favours a disc density profile with λ≈1 and viscosity α between 0.01 and 0.1.

## Abstract

Numerous tidal disruption event (TDE) candidates originating from galactic centres have been detected (e.g., by ${\it Swift}$ 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., ${\it Swift}$ J1644 and ${\it Swift}$ 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 ${\it Swift}$ J1644, ${\it Swift}$ J2058 and ASASSN-14li. Our analysis favours a disc profile with $\lambda\sim1$ for viscosity parameters $\alpha\sim0.01-0.1$.