Observable Signatures of EMRI Black Hole Binaries Embedded in Thin\n Accretion Disks

Abstract

We examine the electromagnetic (EM) and gravitational wave (GW) signatures of\nstellar-mass compact objects (COs) spiraling into a supermassive black hole\n(extreme mass-ratio inspirals or EMRIs), embedded in a thin, radiation-pressure\ndominated, accretion disk. At large separations, the tidal effect of the\nsecondary CO clears a gap. We show that the gap refills during the late\nGW-driven phase of the inspiral, leading to a sudden EM brightening of the\nsource. The accretion disk leaves an imprint on the GW through its angular\nmomentum exchange with the binary, the mass increase of the binary members due\nto accretion, and its gravity. We compute the disk-modified GWs both in an\nanalytical Newtonian approximation and in a numerical effective-one-body\napproach. We find that disk-induced migration provides the dominant\nperturbation to the inspiral, with weaker effects from the mass accretion onto\nthe CO and hydrodynamic drag. Depending on whether a gap is present, the\nperturbation of the GW phase is between 10 and 1000 radians per year,\ndetectable with the future Laser Interferometer Space Antenna (LISA) at high\nsignificance. The Fourier transform of the disk-modified GW in the stationary\nphase approximation is sensitive to disk parameters with a frequency trend\ndifferent from post-Newtonian vacuum corrections. Our results suggest that\nobservations of EMRIs may place new sensitive constraints on the physics of\naccretion disks.\n