Powerful Flares from Recoiling Black Holes in Quasars

Abstract

Mergers of spinning black holes can give recoil velocities fromgravitational radiation up to several thousand km s$^{-1}$.A recoiling supermassive black hole in an AGN retains the inner part ofits accretion disk. Marginally bound material rejoining the disk aroundthe moving black hole releases a large amount of energy in shocks in ashort time, leading to a flare in thermal soft X-rays with a luminosityapproaching the Eddington limit. Reprocessing of the X-rays by theinfalling material gives strong optical and ultraviolet emission lineswith a distinctive spectrum. Despite the short lifetime of the flare(~10$^{4}$ yr), as many as 10$^{2}$ flares may be in play atthe present time in QSOs at redshifts ~1-3. These flares provide a meansto identify high-velocity recoils.