The Electromagnetic Counterparts of Supermassive Black Hole Binaries

The gravitational waves emitted by supermassive black hole binaries at
cosmological distances are expected to be detectable by the future
Laser Interferometric Space Antenna (LISA). While the gravitational
waves themselves will provide a treasure trove of information, if the
source can be identified in electromagnetic bands, this would open up
entirely new scientific opportunities, to probe black hole
astrophysics, cosmology, and novel aspects of gravitational
physics. Any detectable emission from these supermassive black hole
binaries is likely to be time-variable, and I will argue that this may
make the identification feasible. Possibilities for such variable
emission include a roughly periodic signal due to the orbital motion
prior to coalescence, a transient precursor, caused by the gas trapped
inside the binary's orbit, and a transient "after-glow" produced by
merger-induced shocks in the circumbinary disk and by post-merger gas
accretion. I will discuss how these time-variable signatures may be
detectable, and how they can help in identifying a unique counterpart,
even with the relatively poor (fraction of a square degree) sky
localization provided by LISA.