Multi-messenger approaches to binary supermassive black holes in the
  "continuous-wave" regime
  Pulsar timing arrays are sensitive to gravitational waves from supermassive
black hole (SMBH) binaries at orbital separations of << 1pc. There is currently
an observational paucity of such systems, although they are central figures in
studies of galaxy evolution, merger dynamics, and active nucleus formation. We
review the prospects of detecting SMBH binaries through electromagnetic
radiative processes thought to be associated with galaxy mergers and late-stage
binary evolution. We then discuss the scientific goals of joint pulsar timing
and electromagnetic studies of these systems, including the facilitation of
binary parameter estimation, identifying galactic hosts of gravitational wave
emitters, and relevant studies of merger dynamics and cosmology. The use of
upcoming high-precision timing arrays with the International Pulsar Timing
Array and the Square Kilometre Array, combined with ongoing electromagnetic
observing campaigns to identify active SMBH binaries, provide generous
possibilities for multi-messenger astrophysics in the near future.
