Gravitational waves generated by null cosmic strings

We show that null cosmic strings disturb gravitational fields of massive bodies and create outgoing gravitational waves (GW). We find perturbations of the metric caused by a straight null string and a pointlike massive source as solutions to linearized Einstein equations on a flat space-time and derive an analytic approximation for their asymptotic at future null infinity. A space-time created by the source and the string is shown to have an asymptotically polyhomogeneous form. We calculate GW flux in such space-times and demonstrate that the averaged intensity of the radiation is maximal in the direction of the string motion. Opportunities to detect null string generated gravity waves are briefly discussed.