A Null Shell in 2+1 Gravity

Using the generalized Kerr-Schild method we have been able to extend the charged BTZ solution including a null field travelling along the horizon. The exterior solution remains unaltered up to a renormalization of the charge. The geodesic structure is displayed and analized at the horizon. The equivalent solution for the 3+1 case and higher dimensions are discussed. Decreasing the dimensionality of the space turns a Dirac delta function to a step function sitting at the horizon.