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 outward as a brown dwarf. Because the time interval between consecutive EMRIs is comparable to the mass-transfer timescale, the semimajor axes of two consecutive mass-transferring EMRIs will cross on a radial scale of less than a 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 gas production events of mass , 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 (TDEs), 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.” Gas produced by EMRI collisions is easier to circularize than the highly eccentric debris streams produced in TDEs. We predict flares with bolometric luminosities that decay both as power laws shallower than and as decaying exponentials in time. Viscous spreading of the gaseous disks produced by the accumulation of previous mass-stripping events will place a substantial mass of gas on radial scales at the time of a given flare, providing a possible explanation for the “reprocessing blanket” required to explain the unexpectedly high optical luminosities of some candidate TDE flares.