Two regimes of tidal-stream circularization by supermassive black holes

Abstract

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 $M_{\bullet }$ and penetration factor $\beta$, the ratio of the tidal radius to the pericenter distance. We uncover two distinct regimes depending on whether the time $t_c$ at which the most tightly bound debris circularizes is greater or less than the time $t_{\mathrm{fb}}$ at which the mass fallback rate peaks. The bolometric light curve of energy dissipated in the stream self-collisions has a single peak at $t>t_{\mathrm{fb}}$ in the slow circularization regime ($t_c>t_{\mathrm{fb}}$), but two peaks (one at $t<t_{\mathrm{fb}}$ and a second at $t_{\mathrm{fb}}$) in the fast circularization regime ($t_c<t_{\mathrm{fb}}$). Tidal streams will circularize in the slow (fast) regime for apsidal precession angles less (greater) than 0.2 radians which occur for $\beta \lesssim (\gtrsim )(M_{\bullet }/10^6{M}_{\odot }{)}^{-2/3}$. 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.

arXiv comment14 pages, 11 figures, submitted to PRD

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