MG11 
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Robinson-Trautman spacetimes in higher dimensions

We investigate Robinson-Trautman spacetimes (which admit a hypersurface orthogonal, non-shearing, but expanding geodesic null congruence) in higher dimensional GR. The complete family of solutions to Einstein's equations with an arbitrary cosmological constant (and possibly aligned pure radiation) is presented. Analogies and differences with the well known D=4 case are discussed. The Weyl tensor is of type II or more special, but type III and N vacuum solutions are forbidden. While static Schwarzschild-Tangherlini black holes and their generalizations emerge as a privileged vacuum subclass (of type D), there is no formal analogue of the C-metric within the higher dimensional class.

Ultrarelativistic boost of the black ring

We investigate the ultrarelativistic boost of five-dimensional vacuum black rings. Following the classical method of Aichelburg and Sexl, we analytically determine the gravitational field generated by a black ring moving ``with the speed of light'' in an arbitrary direction. This is an impulsive pp-wave propagating in Minkowski spacetime. We study in particular two complementary boosts along axes orthogonal and parallel to the plane of rotation, for which the curvature singularity of the original stationary spacetime becomes a singular ring or a rod, respectively, contained within the wave front. The solution is simpler and physically more interesting if one enforces equilibrium between the forces acting on the ring. It reduces to the five-dimensional Aichelburg-Sexl monopole in the limit when the singularity shrinks to a point. The same boosting technique can also be applied to charged and supersymmetric black rings.