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

TL;DR

This paper reveals that traditional single-temperature blackbody models significantly underestimate TDE accretion disc sizes from X-ray spectra and introduces a new fitting method that provides more accurate size estimates without assuming steady state conditions.

Contribution

It identifies systematic biases in current TDE disc size estimations and proposes a new spectral fitting function that improves accuracy without steady state assumptions.

Findings

Single-temperature blackbody models underestimate disc sizes by up to an order of magnitude.

The new fitting function accurately reproduces disc sizes from X-ray spectra.

Inclination and absorption effects cause smaller additional underestimations.

Abstract

The physical sizes of TDE accretion discs are regularly inferred, from the 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 Letter 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. An XSPEC implementation of this new fitting function is available at github.com/andymummeryastro/TDEdiscXraySpectrum.