Prompt Shocks in the Gas Disk around a Recoiling Supermassive Black Hole Binary

Abstract

Supermassive black hole binaries (BHBs) produced in galaxy mergersrecoil at the time of their coalescence due to the emission ofgravitational waves (GWs). We simulate the response of a thin, 2D diskof collisionless particles, initially on circular orbits around a10$^{6}$ M$_{⊙}$ BHB, to kicks that are eitherparallel or perpendicular to the initial orbital plane. Typical kickvelocities (v$_{kick}$) can exceed the sound speed in acircumbinary gas disk. While the inner disk is strongly bound to therecoiling binary, the outer disk is only weakly bound or unbound. Thisleads to differential motions in the disturbed disk that increase withradius and can become supersonic at gtrsim700 Schwarzschild radii forv$_{kick}$ = 500 km s$^{-1}$ implying that shocks formbeyond this radius. We indeed find that kicks in the disk plane lead toimmediate strong density enhancements (within weeks) in a tightly woundspiral caustic, propagating outward at the speed ~v$_{kick}$.Concentric density enhancements are also observed for kicksperpendicular to the disk, but are weaker and develop into caustics onlyafter a long delay ($\gt$1 yr). Unless both BH spins are low or preciselyaligned with the orbital angular momentum, a significant fraction(gtrsim several %) of kicks are sufficiently large and well aligned withthe orbital plane for strong shocks to be produced. The shocks couldresult in an afterglow whose luminosity and characteristic photon energyincreases with time, helping to identify the EM counterparts of GWsources discovered by LISA.