NONEXPANDING IMPULSIVE GRAVITATIONAL-WAVES

We investigate a class of impulsive gravitational waves which propagat e either in Minkowski or in the (anti-) de Sitter background. These wa ves are constructed as impulsive members of the Kundt class P(Lambda) of non-twisting, non-expanding type-N solutions of vacuum Einstein equ ations with a cosmological constant Lambda. We show that the only non- trivial waves of this type in Minkowski spacetime are impulsive pp-wav es. For Lambda not equal 0 we demonstrate that the canonical subclasse s of P(Lambda), which are invariantly different for smooth profiles, a re all locally equivalent for impulsive profiles. Also, we, present a coordinate system for these impulsive solutions which is explicitly co ntinuous.