Tidal disruption event discs are larger than they seem: Removing systematic biases in TDE X-ray spectral modelling

Abstract

The physical sizes of tidal disruption event (TDE) accretion discs are regularly inferred, from modelling of the TDEs X-ray spectrum as a singleerature blackbody, to be smaller than the plausible event horizons of the black holes which they occur around - a clearly unphysical result. In this Lltter, we demonstrate that the use of singleerature blackbody functions results in the systematic underestimation of TDE accretion disc sizes by as much as an order of magnitude. In fact, the radial 'size' inferred from fitting a singleerature blackbody to an observed accretion disc X-ray spectrum does not even positively correlate with the physical size of that accretion disc. We further demonstrate that the disc-observer inclination angle and absorption of X-ray photons may both lead to additional underestimation of the radial sizes of TDE discs, but by smaller factors. To rectify these issues, we present a new fitting function which accurately reproduces the size of an accretion disc from its 0.3-10 keV X-ray spectrum. Unlike traditional approaches, this new fitting function does not assume that the accretion disc has reached a steady-state configuration, an assumption which is unlikely to be satisfied by most TDEs.