Powerful Flares from Recoiling Black Holes in Quasars

Mergers of spinning black holes can give recoil velocities from gravitational radiation up to several thousand km s−1. A recoiling supermassive black hole in an AGN retains the inner part of its accretion disk. Marginally bound material rejoining the disk around the moving black hole releases a large amount of energy in shocks in a short time, leading to a flare in thermal soft X-rays with a luminosity approaching the Eddington limit. Reprocessing of the X-rays by the infalling material gives strong optical and ultraviolet emission lines with a distinctive spectrum. Despite the short lifetime of the flare (~104 yr), as many as 102 flares may be in play at the present time in QSOs at redshifts ~1-3. These flares provide a means to identify high-velocity recoils.