Radio Crickets: Chirping Jets from Black Hole Binaries Entering their Gravitational Wave Inspiral

We study a novel electromagnetic signature of supermassive black hole binaries whose inspiral starts being dominated by gravitational wave (GW) emission. Recent simulations suggest that the binary's member BHs can continue to accrete gas from the circumbinary accretion disk in this phase of the binary's evolution, all the way until coalescence. If one of the binary members produces a radio jet as a result of accretion, the jet precesses along a biconical surface due to the binary's orbital motion. When the binary enters the GW phase of its evolution, the opening angle widens, the jet exhibits milliarcsecond scale wiggles, and the conical surface of jet precession is twisted due to apparant superluminal motion. The rapidly increasing orbital velocity of the binary gives the jet an appearance of a "chirp." This helical chirping morphology of the jet can be used to infer the binary parameters. For binaries with mass 10^7--10^10 Msun at redshifts z<0.5, monitoring these features in current and archival data will place a lower limit on sources that could be detected by eLISA and Pulsar Timing Arrays. In the future, microarcsecond interferometry with the Square Kilometer Array will increase the potential usefulness of this technique.