Black Hole Collisions With Thin Accretion Disks: OJ 287 and Small-Mass-Ratio Supermassive Black Hole Binary Candidates

Abstract

OJ 287 is the best-known supermassive black hole binary candidate in the nanohertz gravitational wave band. It exhibits periodic flares every $\sim$ 12 years, likely caused by collisions of a smaller-mass secondary with the accretion disk surrounding a larger-mass primary. It is therefore an important benchmark for understanding black hole binary accretion in the approaching era of space-based gravitational wave detectors and large electromagnetic surveys. Because the electromagnetic emission of the system is determined by a complex interplay of plasma, accretion, and radiation physics in strong gravity, numerical simulations are required for realistic modeling. We present the first global, three-dimensional, general relativistic magnetohydrodynamic (GRMHD) simulations of OJ 287-like systems; namely, smaller-mass secondaries colliding with a radiatively-cooled (thin) disk surrounding a larger-mass primary. We focus on disks with scale heights that are 10\% of the distance from the primary and binary mass ratios of $q = 0.1,0.05$, and $0.025$ using an optically-thin cooling prescription. We confirm the basic paradigm that impacts of the secondary on the disk can generate enough power to outshine the quiescent emission. The secondary also causes spiral shocks to form in the disk, enhanced accretion events, overall heating of the flow, and stochastic tilting of the disk, though these effects are small for $q<0.05$. Our results can be extrapolated to the parameters of OJ 287 and similar systems, an important step on the path toward fully realistic simulations of accretion onto small-mass-ratio black hole binaries and predicting electromagnetic counterparts to low-frequency gravitational wave detections.