Diamonds in the sky: the shape of gravitational waves from cosmic string cusps

Abstract

We carry an in-depth analysis of the gravitational wave (GW) emission of cosmic string cusps, going beyond the seminal work of Damour and Vilenkin. We identify two features that, to the best of our knowledge, have not been described in the literature: first the GWs emission from a cusp, at any given frequency, is not concentrated inside a cone but instead draws a diamond-like shape in the sky. Second, the emission is not maximal in the direction of the cusp velocity, but in two slightly offset directions. We explain these effects in detail, and illustrate them with simplified examples. Then, we study two limiting cases for which the overall radiated power is enhanced: when the diamond is stretched in a particular direction, and when higher order derivatives vanish at the cusp. We statistically estimate the importance of these effects in a population of odd-harmonic loops.