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

The physical sizes of tidal disruption event (TDE) accretion discs are regularly inferred, from modelling of the TDEs X-ray spectrum as a single-temperature 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 single-temperature 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 single-temperature 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.