Binary black hole accretion during inspiral and merger

We present the results of 2D, moving mesh, viscous hydrodynamical simulations of accretion on to merging supermassive black hole (SMBH) binaries. We include viscous heating, shock heating, and radiative cooling, and simulate the transition from the ‘pre-decoupling’ epoch, where the inspiral time-scale is longer than the viscous time-scale, to the ‘post-decoupling’ epoch, where the inspiral time-scale is shorter than the viscous time-scale. We find that there is no abrupt halt to the accretion at decoupling, but rather the accretion shows a slow decay, with significant accretion well after the expected decoupling. Moreover, we find that the luminosity in X-rays is significantly higher prior to the merger, as orbital energy from the SMBH binary is converted to heat via strong shocks inside the cavity, and radiated away. Following the merger, the cavity refills viscously and the accretion rate relaxes to the Shakura–Sunyaev value, while the X-ray luminosity drops as the shocks quickly dissipate.