Tidal Disruption Encores

Abstract

Nuclear star clusters (NSCs), made up of a dense concentrations of stars and the compact objects they leave behind, are ubiquitous in the central regions of galaxies, surrounding the central supermassive black hole (SMBH). Close interactions between stars and stellar-mass black holes (sBH) lead to tidal disruption events (TDEs). We uncover an interesting new phenomenon: For a subset of these, the unbound debris (to the sBH) remain bound to the SMBH, accreting at a later time, and thus giving rise to a second flare. We compute the rate of such events, and find them ranging within $10^{-6}$ -$10^{-3}$ yr$^{-1}$gal$^{-1}$ for SMBH mass $\simeq 10^{6}-10^{9}M_\odot$. Time delays between the two flares spread over a wide range, from less than a year to hundreds of years. The temporal evolution of the light curves of the second flare can vary between the standard $t^{-5/3}$ power-law to much steeper decays, providing a natural explanation for observed light curves in tension with the classical TDE model. Our predictions have implications for learning about NSC properties and calibrating its sBH population. Some double flares may be electromagnetic counterparts to LISA Extreme-Mass-Ratio-Inspiral (EMRI) sources. Another important implication is the possible existence of TDE-like events in very massive SMBHs, where TDEs are not expected. Such flares can affect spin measurements relying on TDEs in the upper SMBH range.