Twistor initial data characterisation of pp-waves

This note gives a concise derivation of a twistor-initial-data characterisation of pp-wave spacetimes in vacuum. The construction is based on a similar calculation for the Minkowski spacetime in [Class. Quantum Grav. 28 075010]. The key difference is that for the Minkowski spacetime a necessary condition is that $\nabla_{A}{}^{A'}\bar{\kappa}_{A'} \neq 0$. In this note it is shown that if $\nabla_{A}{}^{A'}\bar{\kappa}_{A'}=0$ then the development is a pp-wave spacetime. Furthermore, it is shown that such condition propagates off the initial hypersurface, which, in turn, gives a \emph{twistor initial data characterisation of pp-waves}.