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We consider a geometrically thin, Keplerian disk in the orbital plane of abinary black hole (BHBH) consisting of a spinning primary and low-masssecondary (mass ratio q < 1). To account for the principle effects of generalrelativity (GR), we propose a modification of the standard Newtonian evolutionequation for the (orbit-averaged) time-varying disk surface density. In ourmodified equation the viscous torque in the disk is treated in full GR, whilethe tidal torque is handled in the Newtonian limit. Our GR-hybrid treatment isreasonable because the tidal torque is concentrated near the orbital radius ofthe secondary and is most important prior to binary-disk decoupling, when theorbital separation is large and resides in the weak-field regime. The tidaltorque on the disk diminishes during late merger and vanishes altogetherfollowing merger. By contrast, the viscous torque drives the flow into thestrong-field region and onto the primary during all epochs. Following binarycoalescence, the viscous torque alone governs the time-dependent accretion ontothe remnant, as well as the temporal behavior, strength and spectrum of theaftermath electromagnetic radiation from the disk. We solve our GR-hybridequation for a representative BHBH-disk system, identify several observable EMsignatures of the merger, and compare results obtained for the gas and EMradiation with those found with the Newtonian prescription.

Accretion Disks Around Binary Black Holes: A Simple GR-Hybrid Evolution Model