Searching for gravitational wave memory bursts with the Parkes Pulsar
  Timing Array
  Anisotropic bursts of gravitational radiation produced by events such as
super-massive black hole mergers leave permanent imprints on space. Such
gravitational wave "memory" (GWM) signals are, in principle, detectable through
pulsar timing as sudden changes in the apparent pulse frequency of a pulsar. If
an array of pulsars is monitored as a GWM signal passes over the Earth, the
pulsars would simultaneously appear to change pulse frequency by an amount that
varies with their sky position in a quadrupolar fashion. Here we describe a
search algorithm for such events and apply the algorithm to approximately six
years of data from the Parkes Pulsar Timing Array. We find no GWM events and
set an upper bound on the rate for events which could have been detected. We
show, using simple models of black hole coalescence rates, that this
non-detection is not unexpected.
