  Supermassive black hole binaries are likely to accrete interstellar gas
through a circumbinary disk. Shortly before merger, the inner portions of this
circumbinary disk are subject to general relativistic effects. To study this
regime, we approximate the spacetime metric of close orbiting black holes by
superimposing two boosted Kerr-Schild terms. After demonstrating the quality of
this approximation, we carry out very long-term general relativistic
magnetohydrodynamic simulations of the circumbinary disk. We consider black
holes with spin dimensionless parameters of magnitude 0.9, in one simulation
parallel to the orbital angular momentum of the binary, but in another
anti-parallel. These are contrasted with spinless simulations. We find that,
for a fixed surface mass density in the inner circumbinary disk, aligned spins
of this magnitude approximately reduce the mass accretion rate by 14% and
counter-aligned spins increase it by 45%, leaving many other disk properties
unchanged.
