Electromagnetic signatures of supermassive black hole binaries resolved
  by PTAs
  Pulsar timing arrays (PTAs) may eventually be able to detect not only the
stochastic gravitational-wave (GW) background of SMBH binaries, but also
individual, particularly massive binaries whose signals stick out above the
background. In this contribution, we discuss the possibility of identifying and
studying such `resolved' binaries through their electromagnetic emission. The
host galaxies of such binaries are themselves expected to be also very massive
and rare, so that out to redshifts z~2 a unique massive galaxy may be
identified as the host. At higher redshifts, the PTA error boxes are larger and
may contain as many as several hundred massive-galaxy interlopers. In this
case, the true counterpart may be identified, if it is accreting gas
efficiently, as an active galactic nucleus (AGN) with a peculiar spectrum and
variable emission features. Specifically, the binary's tidal torques expel the
gas from the inner part of the accretion disk, making it unusually dim in X-ray
and UV bands and in broad optical emission lines. The tails of the broad wings
of any FeK-alpha emission line may also be `clipped' and missing. The binary's
orbital motion, as well as the gas motions it induces, may trigger
quasiperiodic variations. These include coherent flux variability, such as
luminous, multi-wavelength flares, as well as Doppler shifts of broad emission
lines and `see-saw' oscillations in the FeK-alpha line. Additional features,
such as evidence for a recent major merger or dual collimated jets, could also
corroborate the counterpart. These properties would make resolved PTA sources
stand out among AGN with similar overall luminosities and allow their
identification.
