Electromagnetic signature of supermassive black hole binaries that enter their gravitational-wave induced inspiral

Abstract

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 less than or similar 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 similar to 1-10 years and an orbital velocity v of a few thousand km s(-1), with a very weak dependence on the supply rate of gas (v proportional to (M) over dot(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. Narrow-band monitoring of the broad emission lines of quasars on time scales of months to decades can set a lower limit on the expected rate of GW sources for the Laser Interferometer Space Antenna.