Triplets of supermassive black holes: Astrophysics, Gravitational Waves
  and Detection
  Supermassive black holes (SMBHs) found in the centers of many galaxies have
been recognized to play a fundamental active role in the cosmological structure
formation process. In hierarchical formation scenarios, SMBHs are expected to
form binaries following the merger of their host galaxies. If these binaries do
not coalesce before the merger with a third galaxy, the formation of a black
hole triple system is possible. Numerical simulations of the dynamics of
triples within galaxy cores exhibit phases of very high eccentricity (as high
as $e \sim 0.99$). During these phases, intense bursts of gravitational
radiation can be emitted at orbital periapsis. This produces a gravitational
wave signal at frequencies substantially higher than the orbital frequency. The
likelihood of detection of these bursts with pulsar timing and the Laser
Interferometer Space Antenna ({\it LISA}) is estimated using several population
models of SMBHs with masses $\gtrsim 10^7 {\rm M_\odot}$. Assuming a fraction
of binaries $\ge 0.1$ in triple system, we find that few to few dozens of these
bursts will produce residuals $>1$ ns, within the sensitivity range of
forthcoming pulsar timing arrays (PTAs). However, most of such bursts will be
washed out in the underlying confusion noise produced by all the other
'standard' SMBH binaries emitting in the same frequency window. A detailed data
analysis study would be required to assess resolvability of such sources.
Implementing a basic resolvability criterion, we find that the chance of
catching a resolvable burst at a one nanosecond precision level is 2-50%,
depending on the adopted SMBH evolution model. On the other hand, the
probability of detecting bursts produced by massive binaries (masses $\gtrsim
10^7\msun$) with {\it LISA} is negligible.
