Unit Killing Initial Data

Abstract

We find a system of differential equations on an initial data surface whose solutions are in bijection with unit vector fields on the Einstein $Λ$-vacuum development that are proportional to a Killing vector. We refer to these conditions as the \textit{unit Killing initial data} (uKID) equations, analogous to the classical \textit{Killing initial data} (KID) equations. The uKID equations can be useful in a setting where only the unit-normalized part of the Killing vector is geometrically distinguished. We eliminate the scaling degree of freedom of a general Killing vector to obtain the space-time equations characterizing unit normalized Killing vector fields and also the uKID equations. These equations are also prolonged to canonical connection form, showing their finite type character. Finally, we obtain an independent derivation of the uKID equations by revisiting the propagation identity method, which has previously been used to characterize the initial data of other geometric equations.