Unequal Mass Binary Evolution Driven by High Mach Circumbinary Disks

Abstract We present a study of the gas-driven orbital evolution of unequal mass black hole binaries with circumbinary gas disks (CBDs), varying Mach number (M) and viscosity (). Using two-dimensional grid-based hydrodynamics simulations spanning a thousand binary orbits at fixed separation, we explore low to moderate mass ratios (q = 0. 05–1. 0) and examine how variations in M and q affect the torques and component accretion rates exerted by the CBD and consequently the binary evolution. Equal mass binary systems receive positive torques in low- M disks but transition to negative torques for M 25. As q decreases, the transition moves to higher Mach numbers. For q 0. 1, we find no torque sign reversal below M 52, except in sufficiently low-viscosity disks. We find that the secondary black hole cannot effectively repel the CBD, it instead accretes most of the inflowing gas from the CBD; these low mass ratio binaries in high viscosity disks therefore tend to outspiral, although inspiral can occur in less viscous environments. We also find that binaries with mass ratios in the range of 0. 25 − 0. 5 can show preferential accretion favoring the primary when the gas viscosity is low, exemplifying an exception to the established rule of thumb that accretion favors the secondary. We discuss differences between our results and those reported in the literature on the orbital evolution and preferential accretion, and emphasize that our simulations extend into a regime that remains largely unexplored. Overall, our results suggest that intermediate mass ratio inspirals (IMRIs) in CBDs may be less frequent, but this depends sensitively on the interplay between mass ratio, disk temperature, and viscosity.