A precise electromagnetic measurement of the sky coordinates and redshift ofa coalescing black hole binary holds the key for using its gravitational wave(GW) signal to constrain cosmological parameters and to test generalrelativity. Here we show that the merger of ~10^{6-7}M_sun black holes isgenerically followed over a period of years by multiple electromagnetic flaresfrom tidally disrupted stars. The sudden recoil imparted to the merged blackhole by GW emission promptly fills its loss cone and results in a tidaldisruption rate of stars as high as ~0.1 per year. The prompt disruption of astar within a single galaxy over a short period provides a uniqueelectromagnetic flag of a recent black hole coalescence event, and sequentialdisruptions could be used on their own to calibrate the expected rate of GWsources for pulsar timing arrays or the proposed Laser Interferometer SpaceAntenna (LISA).
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