LAUSR

Stars that approach a supermassive black hole (SMBH) too closely can be disrupted by the tidal gravitational field of the SMBH. The resulting debris forms a tidal stream orbiting the SMBH which can collide with itself due to relativistic apsidal precession. These self-collisions dissipate energy, causing the stream to circularize. We perform kinematic simulations of these stream self-collisions to estimate the efficiency of this circularization as a function of SMBH mass M• and penetration factor β, the ratio of the tidal radius to the pericenter distance. We uncover two distinct regimes depending on whether the time tc at which the most tightly bound debris circularizes is greater or less than the time tfb at which the mass fallback rate peaks. The bolometric light curve of energy dissipated in the stream self-collisions has a single peak at t > tfb in the slow circularization regime (tc > tfb), but two peaks (one at t < tfb and a second at tfb) in the fast circularization regime (tc < tfb). Tidal streams will circularize in the slow (fast) regime for apsidal precession angles less (greater) than 0.2 radians which occur for β . (&)(M•/10M )−2/3. The observation of prominent double peaks in bolometric TDE light curves near the transition between these two regimes would strongly support our model of tidal-stream kinematics.