Brightening of an Accretion Disk Due to Viscous Dissipation of   Gravitational Waves During the Coalescence of Supermassive Black Holes
  Mergers of supermassive black hole binaries release peak power of up to ~10^57 erg/s in gravitational waves (GWs). As the GWs propagate through ambient gas, they induce shear and a small fraction of their power is dissipated through viscosity. The dissipated heat appears as electromagnetic (EM) radiation, providing a prompt EM counterpart to the GW signal. For thin accretion disks, the GW heating rate exceeds the accretion power at distances farther than 10^3 Schwarzschild radii, independently of the accretion rate and viscosity coefficient. 
Bence Kocsis, Abraham Loeb (Harvard)