  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.
