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

Abstract

Using a one-dimensional height integrated model, we calculate the evolution\nof an unequal mass binary black hole with a coplanar gas disk that contains a\ngap due to the presence of the secondary black hole. Viscous evolution of the\nouter circumbinary disk initially hardens the binary, while the inner disk\ndrains onto the primary (central) black hole. As long as the inner disk remains\ncool and thin at low $\\dot{M}_{\\rm ext}$ (rather than becoming hot and\ngeometrically thick), the mass of the inner disk reaches an asymptotic mass\ntypically $\\sim 10^{-3}-10^{-4}\\Msun$. Once the semimajor axis shrinks below a\ncritical value, angular momentum losses from gravitational waves dominate over\nviscous transport in hardening the binary. The inner disk then no longer\nresponds viscously to the inspiraling black holes. Instead, tidal interactions\nwith the secondary rapidly drive the inner disk into the primary. Tidal and\nviscous dissipation in the inner disk lead to a late time brightening in\nluminosity $L\\propto t_{\\rm minus}^{5/4}$, where $t_{\\rm minus}$ is the time\nprior to the final merger. This late time brightening peaks $\\sim 1$ day prior\nto the final merger at $\\sim 0.1 L_{\\rm Edd}$. This behavior is relatively\nrobust because of self regulation in the coupled viscous-gravitational\nevolution of such binary systems. It constitutes a unique electromagnetic\nsignature of a binary supermassive black hole merger and may allow the host\ngalaxy to be identified if used in conjunction with the Laser Interferometric\nSpace Antenna (LISA) localization.\n