Multimessenger astronomy with pulsar timing and X-ray observations of
  massive black hole binaries
  We demonstrate that very massive (>10^8\msun), cosmologically nearby (z<1)
black hole binaries (MBHBs), which are primary targets for ongoing and upcoming
pulsar timing arrays (PTAs), are particularly appealing multimessenger
carriers. According to current models for massive black hole formation and
evolution, the planned Square Kilometer Array (SKA) will collect gravitational
wave signals from thousands of such massive systems, being able to individually
resolve and locate in the sky several of them (maybe up to a hundred). By
employing a standard model for the evolution of MBHBs in circumbinary discs,
with the aid of dedicated numerical simulations, we characterize the gas-binary
interplay, identifying possible electromagnetic signatures of the PTA sources.
We concentrate our investigation on two particularly promising scenarios in the
high energy domain, namely, the detection of X-ray periodic variability and of
double broad K\alpha iron lines. Up to several hundreds of periodic X-ray
sources with a flux >10^-13 erg s^-1 cm^-2 will be in the reach of upcoming
X-ray observatories. Double relativistic K\alpha lines may be observable in a
handful of low redshift (z<0.3) sources by proposed deep X-ray probes, such as
Athena. (Abridged)
