The dynamics and electromagnetic signatures of accretion in unequal mass binary black hole inspirals

We present a theoretical study of the gravitational wave (GW) driven inspirals of accreting black hole binaries with mass $M = 10^7 M_\odot$ and mass ratios between $10^{-3}$ and $10^{-1}$. Our results are based on analytic estimates, and grid-based hydrodynamics simulations run for many thousands of binary orbits before the merger. We show that the GW inspiral is evident in the light curves and color evolution of a binary-hosting quasar, over years to decades before a merger. The long-term electromagnetic (EM) signature is characterized by a gradual UV brightening, and X-ray dimming, followed by an X-ray disappearance hours to days before the GW burst, and finally a years-like re-brightening as the disk relaxes and refuels the remnant black hole. These timescales are surprisingly insensitive to the amplitude of viscous stress in the disk. The spectrum of quasi-thermal disk emission shows two peaks: one in the UV, and another in the X-ray, associated with the outer and circum-secondary disks respectively; emission from the inner disk is suppressed because the secondary consumes most of the inflowing gas. We discuss implications for real-time and archival EM followup of GW bursts detected by LISA.