centra

The recent breakthroughs in numerical relativity
have made it possible to generate the gravitational wave signals
coming from binary black hole systems. The waveforms are
are extracted using the Newman-Penrose formalism and in
particular the Weyl scalar Psi4. In order for Psi4 to contain the physical
information on the gravitational wave signal, the tetrad chosen
for its calculation should converge to the Kinnersley tetrad
in the limit of single black hole space-time. Here we present a compact
expression for Psi4 that automatically enforces this condition, making
it a useful and simple to implement  formula
for wave extraction in numerical relativity.