Fossil gas and the electromagnetic precursor of supermassive binary black hole mergers

Using a 1D height integrated model, we calculate the evolution of an unequal mass binary black hole with a coplanar gas disc that contains a gap due to the presence of the secondary black hole. Viscous evolution of the outer circumbinary disc initially hardens the binary, while the inner disc drains on to the primary (central) black hole. As long as the inner disc remains cool and thin at low (M) over dot(ext) (rather than becoming hot and geometrically thick), the mass of the inner disc reaches an asymptotic mass typically similar to 10(-3)-10(-4) M-circle dot. Once the semimajor axis shrinks below a critical value, angular momentum losses from gravitational waves dominate over viscous transport in hardening the binary. The inner disc then no longer responds viscously to the inspiraling black holes. Instead, tidal interactions with the secondary rapidly drive the inner disc into the primary. Tidal and viscous dissipation in the inner disc lead to a late time brightening in luminosity, L proportional to t(minus)(-5/4), where t(minus) is the time prior to the final merger. This late time brightening peaks similar to 1 d prior to the final merger at similar to 0.1 L-Edd. This behaviour is relatively robust because of self-regulation in the coupled viscous-gravitational evolution of such binary systems. It constitutes a unique electromagnetic signature of a binary supermassive black hole merger and may allow the host galaxy to be identified if used in conjunction with the Laser Interferometric Space Antenna localization.