The X-ray variability of tidal disruption events

ABSTRACT When a star is torn apart by the tidal forces of a supermassive black hole (a so-called tidal disruption events, TDE) a transient accretion episode is initiated and a hot, often X-ray bright, accretion disc is formed. Like any accretion flow this disc is turbulent, and therefore the emission from its surface will vary stochastically. As the disc has a finite mass supply (i.e. at most the initial mass of the disrupted star) the disc will also undergo long-time-scale evolution, as this material is lost into the black hole. In this paper, we combine theoretical models for this long time evolution of the disc with models for the stochastic variability of turbulent accretion flows which are correlated on short (orbital) time-scales. This new framework allows us to demonstrate that (i) dimming events should be more prevalent than brightening events in long-term TDE X-ray light curves (i.e. their log-luminosity distribution should be asymmetric with a skew towards lower luminosities), (ii) TDE X-ray light curves should follow a near- (but formally sub-)linear correlation between their root mean square variability and the mean flux, (iii) the fractional variability observed on short time-scales across an X-ray observing band should increase with observing energy, and (iv) TDEs offer a unique probe of the physics of disc turbulence, owing to their clean spectra and natural evolutionary time-scales. We confirm predictions (i) and (ii) with an analysis of the long time-scale variability of two observed TDEs, and show strong support for prediction (iii) using the intra-observation variability of the same two sources.