Abstract

The gravitational waves from a binary black hole with masses \ can be detected with the Laser Interferometer Space Antenna once their orbital frequency exceeds 10\–10\ Hz. The binary separation at this stage is \R_{\mathrm{g}}\), and the orbital speed is \\). I argue that at this stage, the binary will be producing bright electromagnetic 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 difference \ in the propagation speed of photons and gravitons as low as \.