Circumbinary Disks: Accretion and Torque as a Function of Mass Ratio and
  Disk Viscosity
  Using numerical hydrodynamics calculations and a novel method for densely
sampling parameter space, we measure the accretion and torque on a binary
system from a circumbinary disk. In agreement with previous studies, we find
that the net torque on the binary is positive for mass ratios close to unity,
and that accretion always drives the binary towards equal mass. Accretion
variability depends sensitively on the numerical sink prescription, but the
torque and relative accretion onto each component do not depend on the sink
timescale. Positive torque and highly variable accretion occurs only for mass
ratios greater than around $0.05$. This means that for mass ratios below
$0.05$, the binary would migrate inward until the secondary accreted sufficient
mass, after which it would execute a U-turn and migrate outward. We explore a
range of viscosities, from $\alpha = 0.03$ to $\alpha = 0.15$, and find that
this outward torque is proportional to the viscous torque, simply proportional
to viscosity in this range. Dependence of accretion and torque on mass ratio is
explored in detail, densely sampling mass ratios between $0.01$ and unity. For
mass ratio $q > 0.6$, accretion variability is found to exhibit a distinct
sawtooth pattern, typically with a five-orbit cycle that provides a "smoking
gun" prediction for variable quasars observed over long periods, as a potential
means to confirm the presence of a binary.
