The gravitational waves ( GWs ) from a binary black hole ( BBH ) with masses 10 ^ { 4 } \mathrel { \hbox to 0.0 pt { \lower 4.0 pt \hbox { $ \sim$ } } \raise 1.0 pt \hbox { $ < $ } % } M \mathrel { \hbox to 0.0 pt { \lower 4.0 pt \hbox { $ \sim$ } } \raise 1.0 pt \hbox { $ < $ } } 10 ^ % { 7 } { M _ { \odot } } 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 ) R _ { g } ( 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 difference \Delta v in the propagation speed of photons and gravitons as low as \Delta v / c \approx 10 ^ { -17 } .