Pre-merger Localization of Gravitational-wave Standard Sirens with Lisa: Triggered Search for an Electromagnetic Counterpart

Electromagnetic (EM) counterparts to supermassive black hole binary mergers observed by LISA can be localized to within the field of view of astronomical instruments (∼ 10deg) hours to weeks prior to coalescence. The temporal coincidence of any prompt EM counterpart with a gravitationally-timed merger may offer the best chance of identifying a unique host galaxy. We discuss the challenges posed by searches for such prompt EM counterparts and propose novel observational strategies to address them. In particular, we discuss the size and shape evolution of the LISA localization error ellipses on the sky, and quantify the corresponding requirements for dedicated EM surveys of the area prior to coalescence. The basic requirements of a wide field of view and fast detectors are similar to those for searches being planned for distant cosmological supernovae. A triggered EM counterpart search campaign will require monitoring a several–square degree area. It could aim for variability at the 24–27 mag level in optical bands, for example, which corresponds to 1-10% of the Eddington luminosity of the prime LISA sources of ∼ (10–10)M⊙ BHs at z = 1 − 2, on time–scales of minutes to hours, the orbital time–scale of the binary in the last 2–4 weeks of coalescence. A cross–correlation of the period of any variable EM signal with the quasi–periodic gravitational waveform over 10-1000 cycles may aid the detection. Alternatively, EM searches can detect a transient signal accompanying the coalescence. The triggered searches will be ambitious, but if they successfully identify a unique prompt electromagnetic counterpart, they will enable new fundamental tests of gravitational physics. We highlight the measurement of differences in the arrival times of photons and gravitons from the same cosmological source as a valuable independent test of the massive character of gravity, and of possible violations of Lorentz invariance in the gravity sector.