An Electromagnetic Signature of Galactic Black Hole Binaries That Enter
  Their Gravitational-Wave Induced Inspiral
  Mergers of gas-rich galaxies lead to black hole binaries that coalesce as a
result of dynamical friction on the ambient gas. Once the binary tightens to
<10^3 Schwarzschild radii, its merger is driven by the emission of
gravitational waves (GWs). We show that this transition occurs generically at
orbital periods of ~1-10 years and an orbital velocity V of a few thousand
km/s, with a very weak dependence on the supply rate of gas (V proportional to
Mdot^{1/8}). Therefore, as binaries enter their GW-dominated inspiral, they
inevitably induce large periodic shifts in the broad emission lines of any
associated quasar(s). The probability of finding a binary in tighter
configurations scales as V^{-8} owing to their much shorter lifetimes.
Systematic monitoring of the broad emission lines of quasars on timescales of
months to decades can set a lower limit on the expected rate of GW sources for
LISA.
