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Using numerical hydrodynamics calculations and a novel method for denselysampling parameter space, we measure the accretion and torque on a binarysystem from a circumbinary disk. In agreement with previous studies, we findthat the net torque on the binary is positive for mass ratios close to unity,and that accretion always drives the binary towards equal mass. Accretionvariability depends sensitively on the numerical sink prescription, but thetorque and relative accretion onto each component do not depend on the sinktimescale. Positive torque and highly variable accretion occurs only for massratios greater than around $0.05$. This means that for mass ratios below$0.05$, the binary would migrate inward until the secondary accreted sufficientmass, after which it would execute a U-turn and migrate outward. We explore arange of viscosities, from $\alpha = 0.03$ to $\alpha = 0.15$, and find thatthis outward torque is proportional to the viscous torque, simply proportionalto viscosity in this range. Dependence of accretion and torque on mass ratio isexplored in detail, densely sampling mass ratios between $0.01$ and unity. Formass ratio $q > 0.6$, accretion variability is found to exhibit a distinctsawtooth pattern, typically with a five-orbit cycle that provides a "smokinggun" prediction for variable quasars observed over long periods, as a potentialmeans to confirm the presence of a binary.

Circumbinary Disks: Accretion and Torque as a Function of Mass Ratio and Disk Viscosity