  We present fully relativistic predictions for the electromagnetic emission
produced by accretion disks surrounding spinning and nonspinning supermassive
binary black holes on the verge of merging. We use the code Bothros to
post-process data from 3D general relativistic magnetohydrodynamic simulations
via ray-tracing calculations. These simulations model the dynamics of a
circumbinary disk and the mini-disks that form around two equal-mass black
holes orbiting each other at an initial separation of 20 gravitational radii,
and evolve the system for more than 10 orbits in the inspiral regime. We model
the emission as the sum of thermal blackbody radiation emitted by an optically
thick accretion disk and a power-law spectrum extending to hard X-rays emitted
by a hot optically thin corona. We generate time-dependent spectra, images, and
light curves at various frequencies to investigate intrinsic periodic signals
in the emission, as well as the effects of the black hole spin. We find that
prograde black hole spin makes mini-disks brighter since the smaller innermost
stable circular orbit angular momentum demands more dissipation before matter
plunges to the horizon. However, compared to mini-disks in larger separation
binaries with spinning black holes, our mini-disks are less luminous: unlike
those systems, their mass accretion rate is lower than in the circumbinary
disk, and they radiate with lower efficiency because their inflow times are
shorter. Compared to a single black hole system matched in mass and accretion
rate, these binaries have spectra noticeably weaker and softer in the UV.
Finally, we discuss the implications of our findings for the potential
observability of these systems.
