Accretion discs in (repeating) partial tidal disruption events: rapid state transitions, UV plateaus, and flares from disc–remnant collisions

ABSTRACT Tidal disruption events (TDEs) which repeat on time-scales of months to years represent an unambiguous signature of a partial disruption, with the surviving stellar remnant returning to pericentre to be repeatedly stripped by tidal forces. These systems therefore offer the best laboratories to study the differences between partial and full disruptions. One noteworthy observational difference between the two systems is that all known X-ray bright repeating TDEs show rapid transitions between thermal, non-thermal and completely dim states on time-scales much shorter than full (non-repeating) TDEs. We argue this can be simply understood as being due to the reduction in fuel supply available to the disc, and that these systems provide evidence that all tidal disruption events undergo a state transition at Eddington ratios $f_{\rm edd} = L_{\rm bol}/L_{\rm edd} \sim 0.01$, similar to X-ray binaries. As part of this calculation we derive a general expression for the time taken for a TDE disc to fall to a given Eddington fraction, which will be of use to both full and partial TDE science. Perhaps surprisingly, the late-time optical/UV plateau luminosity observed from these systems is largely unaffected by this reduction in fuel supply, provided the outer disc remains in a thermal state for long enough for this emission to be detected. We then show that collisions between the returning stellar remnant and the disc formed from the last passage will produce potentially observable X-ray flares ($L_{\rm flare} \simeq 10^{42}$ erg s−1), but that they are likely to be very difficult to detect as they are generally short-lived ($t_{\rm flare} \simeq 0.1\mathrm{ -}1$ h).