Junctions and thin shells in general relativity using computer algebra: II. The null formalism

Abstract

We present extensions to the <TT> GRJunction</TT> computer algebra program, which allow the study of null boundary surfaces and thin shells in general relativity. We summarize the null formalism due to Barrabès and Israel and highlight those steps which differ from the timelike/spacelike cases. <TT> GRJunction</TT> has been used to verify a number of results from the literature. We then present two new results calculated with the aid of <TT> GRJunction</TT>. These are the junction of two Kerr - Newman solutions at a non-horizon straddling null shell in the slow rotation limit and the exact junction of two Kerr - Newman solutions at a horizon straddling shell.