Abstract
ABSTRACT We extend the relativistic time-dependent thin-disc TDE model to describe non-thermal (2−10 keV) X-ray emission produced by the Compton up-scattering of thermal disc photons by a compact electron corona, developing analytical and numerical models of the evolving non-thermal X-ray light curves. In the simplest cases, these X-ray light curves follow power-law profiles in time. We suggest that TDE discs act in many respects as scaled-up versions of XRB discs, and that such discs should undergo state transitions into harder accretion states. XRB state transitions typically occur when the disc luminosity becomes roughly one per cent of its Eddington value. We show that if the same is true for TDE discs then this, in turn, implies that TDEs with non-thermal X-ray spectra should come preferentially from large-mass black holes. The characteristic hard-state transition mass is MHS ≃ 2 × 107M⊙. Hence, subpopulations of thermal and non-thermal X-ray TDEs should come from systematically different black hole masses. We demonstrate that the known populations of thermal and non-thermal X-ray TDEs do indeed come from different distributions of black hole masses. The null-hypothesis of identical black hole mass distributions is rejected by a two-sample Anderson-Darling test with a p-value <0.01. Finally, we present a model for the X-ray rebrightening of TDEs at late times as they transition into the hard state. These models of evolving TDE light curves are the first to join both thermal and non-thermal X-ray components in a unified scenario.
Highlights
The disruption of a star by the gravitational tidal force of a supermassive black hole, a so-called tidal disruption event (TDE), can produce bright flares from otherwise quiescent galactic nuclei
While models of the lower energy emission often invoke outflows launched in the earliest stages of the TDE (e.g. Metzger & Stone 2016, Strubbe & Quataert 2009), or, especially for radio emission, a jet, high energy emission (UV and X-ray), is much more likely to result from an optically thick accretion disc which forms from the disrupted stellar debris
Not all X-ray bright TDEs are dominated by emission from an accretion disc, ; the emission of the brightest X-ray sources almost certainly come from a relativistic jet (Burrows et al 2011)
Summary
The disruption of a star by the gravitational tidal force of a supermassive black hole, a so-called tidal disruption event (TDE), can produce bright flares from otherwise quiescent galactic nuclei. While some TDEs (e.g., five recently discovered sources in van Velzen et al 2021) have Xray spectra which are well-described by quasi-thermal (blackbody superposition) spectra, others (e.g., XMMSL1 J0619, Saxton et al 2014) have X-ray spectra which are well-modelled by nonthermal “hard-state” emission, characterised by a power-law with energy dependence. Sources of this spectral type have been modelled by applying a sequence of steady-state AGN models to each individual observation
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