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The gravitational waves (GWs) from a binary black hole (BBH) with masses 10^410^7 M_ can be detected with the Laser Interferometer Space Antenna (LISA) once their orbital frequency exceeds 10^-4--10^-5 Hz. The binary separation at this stage is a=O (100) R₆ (gravitational radius), and the orbital speed is v/c=O (0. 1). We argue that at this stage, the binary will be producing bright electromagnetic (EM) radiation via gas bound to the individual BHs. Both BHs will have their own photospheres in x-ray and possibly also in optical bands. Relativistic Doppler modulations and lensing effects will inevitably imprint periodic variability in the EM light curve, tracking the phase of the orbital motion, and serving as a template for the GW inspiral waveform. Advanced localization of the source by LISA weeks to months prior to merger will enable a measurement of this EM chirp by wide-field x-ray or optical instruments. A comparison of the phases of the GW and EM chirp signals will help break degeneracies between system parameters, and probe a fractional difference in the propagation speed of photons and gravitons as low as /c10^-17.
Zoltán Haiman (Fri,) studied this question.