IMPULSIVE WAVES IN DE SITTER AND ANTI-DE SITTER SPACETIMES GENERATED BY NULL PARTICLES WITH AN ARBITRARY MULTIPOLE STRUCTURE

We describe a class of impulsive gravitational waves which propagate e ither in a de Sitter or an anti-de Sitter background. They are conform al to impulsive waves of Kundt's class. In a background with positive cosmological constant they are spherical (but nonexpanding) waves gene rated by pairs of particles with arbitrary multipole structure propaga ting in opposite directions. When the cosmological constant is negativ e, they are hyperboloidal waves generated by a null particle of the sa me type. In this case, they are included in the impulsive limit of a c lass of solutions described by Siklos that are conformal to pp-waves.