Periodic accretion-powered flares from colliding EMRIs as TDE Imposters

Abstract

When a main sequence star undergoes Roche lobe overflow onto a supermassive black hole (SMBH) in a circular extreme mass ratio inspiral (EMRI), a phase of steady mass transfer ensues. Over millions of years, the binary evolves to a period minimum before reversing course and migrating outwards. Because the time interval between consecutive EMRIs is comparable to the mass-transfer timescale, the semi-major axes of two consecutive mass-transferring EMRIs will cross on a radial scale < few AU. We show that such EMRI crossing events are inevitably accompanied by a series of mildly relativistic, grazing physical collisions between the stars. Each collision strips a small quantity of mass, primarily from the more massive star, which generally increases their radial separation to set up the next collision after a delay of decades to centuries (or longer) set by further gravitational radiation. Depending on the mass of the SMBH, this interaction can result in N ~ 1-1e4 gas production events of mass Msun/N, thus powering a quasi-periodic sequence of SMBH accretion-powered flares over a total duration of thousands of years or longer. Although the EMRI rate is 2-3 orders of magnitude lower than the rate of tidal disruption events (TDE), the ability of a single interacting EMRI pair to produce a large number of luminous flares - and to make more judicious use of the available stellar fuel - could make their observed rate competitive with the TDE rate, enabling them to masquerade as "TDE Imposters." We predict flares with luminosities that decay both as power laws shallower than t^(-5/3) or as decaying exponentials. Viscous spreading of the gas disks produced by the accumulation of previous mass-stripping events places substantial mass on radial scales > 10-100 AU, providing a reprocessing source required to explain the unexpectedly high optical luminosities of some flares.