Gravitational-wave background from kink-kink collisions on infinite cosmic strings

We calculate the power spectrum of the stochastic gravitational-wave (GW) background expected from kink-kink collisions on infinite cosmic strings. Intersections in the cosmic string network continuously generate kinks, which emit GW bursts by their propagation on curved strings as well as by their collisions. First, we show that the GW background from kink-kink collisions is much larger than the one from propagating kinks at high frequencies because of the higher event rate. We then propose a method to take into account the energy loss of the string network by GW emission as well as the decrease of the kink number due to the GW backreaction. We find that these effects reduce the amplitude of the GW background at high frequencies and produce a flat spectrum. Finally, we obtain a constraint on the string tension of G mu less than or similar to 10(-5) using the current upper bound on the GW background by Advanced LIGO, and G mu less than or similar to 4 x 10(-8) using pulsar timing arrays.