A unified model of tidal destruction events in the disc-dominated phase

Abstract

We develop a unification scheme which explains the varied observed properties of TDEs in terms of simple disc physics. The unification scheme postulates that the different observed properties of TDEs are controlled by the peak Eddington ratio of the accretion discs which form following a stellar disruption. Our primary result is that the TDE population can be split into four subpopulations, which are (in order of decreasing peak Eddington ratio): "obscured" UV-bright and X-ray dim TDEs; X-ray bright soft-state TDEs; UV-bright and X-ray dim "cool" TDEs; and X-ray bright hard-state TDEs. These 4 subpopulations of TDEs will occur around black holes of well defined masses, and our unification scheme is therefore directly testable with observations. As an initial test, we model the X-ray and UV light curves of six TDEs taken from three of the four subpopulations: ASASSN-14ae, ASASSN-15oi, ASASSN-18pg, AT2019dsg, XMMSL1 J0740 & XMMSL2 J1446. We show that all six TDEs, spanning a wide range of observed properties, are well modelled by evolving relativistic thin discs. The peak Eddington ratio's of the six best-fitting disc solutions lie exactly as predicted by the unified model. The mean stellar mass of the six sources is $\left\langle M_\star \right\rangle \sim 0.24 M_\odot$. The so-called `missing energy problem' is resolved by demonstrating that only $\sim 1\%$ of the radiated accretion disc energy is observed at X-ray and UV frequencies. Finally, we present an empirical, approximately linear, relationship between the total radiated energy of the accretion disc and the total radiated energy of an early-time, rapidly-decaying, UV component, seen in all TDEs.