Misaligned snowplough effect and the electromagnetic counterpart to black hole binary mergers

Abstract

We estimate the accretion rates produced when a circumprimary gas disc is pushed into the primary supermassive black hole (SMBH) by the tidal force of the decaying secondary during a SMBH merger. Using the 3D Smoothed Particle Hydrodynamics (SPH) code PHANTOM, we extend previous investigations of co-planar discs to the case where the disc and binary orbital planes are misaligned. We consider a geometrically thin disc with inclination angles varying from 1° to 180° and a binary with mass ratio q = 10-3. We find that discs with small inclination angles (<10°) produce an increase in luminosity exceeding the Eddington rate. By contrast, discs with inclinations between 20° and 30° show a less pronounced rise in the accretion rate, whilst discs inclined by 180° show no peak in the mass accretion rate. While previous analytic work predicted that the effective tidal torque drops with increasing inclination angle, we show that the misaligned snowplough effect remains important even for angles larger than the disc aspect ratio. The rise in the accretion rate produced by discs inclined at small angles to the binary orbit can produce an electromagnetic counterpart to the gravitational wave signal emitted from final stages of the binary orbital decay.