From X-rays to physical parameters: a comprehensive analysis of thermal tidal disruption event X-ray spectra

Abstract

We perform a comprehensive analysis of a population of 19 X-ray bright tidal disruption events (TDEs), fitting their X-ray spectra with a new, physically self consistent, relativistic accretion disc model. Not all of the TDEs inhabit regions of parameter space where the model is valid, or have sufficient data for a detailed analysis, and physically interpretable parameters for a sub-sample of 11 TDEs are determined. These sources have thermal (power-law free) X-ray spectra. The radial sizes measured from these spectra lie at values consistent with the inner-most stable circular orbit of black holes with masses given by the $M_{\mathrm{BH}}-\sigma$ relationship, and can be used as an independent measurement of $M_{\mathrm{BH}}$. The bolometric disc luminosity can also be inferred from X-ray data. All of the TDEs have luminosities which are sub-Eddington ($L_{\mathrm{bol},\mathrm{disc}}\lesssim L_{\mathrm{edd}}$), and larger than the typical hard-state transitional luminosity of X-ray binary discs ($L_{\mathrm{bol},\mathrm{disc}}\gtrsim 0.01L_{\mathrm{edd}}$). The {\it peak} bolometric luminosity is found to be linearly correlated with the $M_{\mathrm{BH}}-\sigma$ mass. The TDE X-ray-to-bolometric correction can reach values up to $\sim 100$, and grows exponentially at late times, resolving the missing energy problem. We show that the peak disc luminosities of some TDEs are smaller than their observed optical luminosities, implying that not all of the early time optical emission can be sourced from reprocessed disc emission. Our results are supportive of the hypothesis that thermal X-ray bright TDEs are in accretion states analogous to the ``soft'' accretion state of X-ray binaries, and that black hole accretion processes are scale (mass) invariant.

17 pages + 3 appendices. 18 figures, 5 tables. Version as accepted for publication in MNRAS

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