An Electromagnetic Signature of Galactic Black Hole Binaries That Enter Their Gravitational-Wave Induced Inspiral

TL;DR

Galactic black hole binaries entering their gravitational-wave driven inspiral produce observable periodic shifts in quasar emission lines, providing a new electromagnetic signature to identify potential LISA sources.

Contribution

This work predicts a universal electromagnetic signature of black hole binaries during their GW-driven inspiral phase, linking GW emission to observable quasar emission line shifts.

Findings

Binary transition to GW-driven inspiral occurs at ~1-10 year orbital periods.

Large periodic shifts in broad emission lines are induced by binaries entering GW inspiral.

Monitoring quasar emission lines can estimate the rate of GW sources for LISA.

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 <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.