Post-periapsis Pancakes: Sustenance for Self-gravity in Tidal Disruption Events

A Tidal Disruption Event (TDE), which occurs when a star is destroyed by the gravitational field of a supermassive black hole, produces a stream of debris, the evolution of which ultimately determines the observational properties of the event. Typically, investigations of this process resort to predicting future accretion of material assuming the debris orbits are Keplerian or simulating restricted parameter regimes where the computational cost is a small fraction of that for more realistic parameters. In a series of papers that took advantage of Complexity@DiRAC, we simulated the long-term evolution of the debris stream for realistic TDE parameters, revealing new effects that can significantly affect the observable properties of these events (Coughlin & Nixon 2015; Coughlin, Nixon et al. 2016a,b). The figure to the right is taken from Coughlin, Nixon et al. (2016a: MNRAS 455, 4, 3612). In this work, we showed that a post-periapsis caustic – a location where the locus of gas parcels comprising the stream would collapse into a two-dimensional surface if they evolved solely in the gravitational field of the hole – occurs when the pericentre distance of the star is of the order of the tidal radius of the hole. We showed that this ‘pancake’ induces significant density perturbations in the debris stream, and these fluctuations can be sufficient to gravitationally destabilize the stream, resulting in its fragmentation into bound clumps.

Figure 1. Column density rendering of the stream of debris produced by a Tidal Disruption Event, showing both the full stream (left) and a zoom-in (right). In this case stream is strongly self-gravitating, and fragments into bound clumps (Coughlin, Nixon et al., 2016).

As these clumps fall back towards the black hole their accretion can result in strong bursts of radiation, while the low-density regions between clumps can yield relatively little. This makes the luminosity of the event highly time variable, and may be an explanation for the rapid time variability of systems such as Swift J1644. However, the detailed behaviour of the clumps is not yet well understood, as it depends on their internal thermodynamics and how quickly they can cool and condense. If they can cool fast enough, they may be able to survive a second pericentre passage and potentially form planets orbiting the black hole, otherwise they will be re-disrupted at pericentre and join the accretion flow. These processes will be the subject of future DiRAC simulations. Over the next few years new observing missions will provide a substantial increase in the number of events for which we have data. Determining how the luminosity of a TDE varies with time is critical to interpreting these events and discerning the progenitor properties, such as black hole mass and spin.