  Pulsar timing arrays (PTAs) are Galactic-scale gravitational wave (GW)
detectors consisting of precisely-timed pulsars distributed across the sky.
Within the decade, PTAs are expected to detect the nanohertz GWs emitted by
close-separation supermassive black hole binaries (SMBHBs), thereby opening up
the low frequency end of the GW spectrum for science. Individual SMBHBs which
power active galactic nuclei are also promising multi-messenger sources; they
may be identified via theoretically predicted electromagnetic (EM) signatures
and be followed up by PTAs for GW observations. In this work, we study the
detection and parameter estimation prospects of a PTA which targets EM-selected
SMBHBs. Adopting a simulated Galactic millisecond pulsar population, we
envisage three different pulsar timing campaigns which observe three mock
sources at different sky locations. We find that an all-sky PTA which times the
best pulsars is an optimal and feasible approach to observe EM-selected SMBHBs
and measure their source parameters to high precision (i.e., comparable to or
better than conventional EM measurements). We discuss the implications of our
findings in the context of the future PTA experiment with the planned Deep
Synoptic Array-2000 and the multi-messenger studies of SMBHBs such as the
well-known binary candidate OJ 287.
