Axion-like particle production in neutron star mergers: gamma-ray signal detectability

Axion-like particles can in principle be produced in the dense environment of neutron star mergers and core-collapse supernovae, and converted in gamma-rays in the magnetic field intervening between the event and the Earth. While this process has been studied for supernovae, there is no estimate of the gamma-ray signal expected from neutron star mergers. In this work we explore this new, exotic signature of the merger, and find that for a large region of the ALP parameter space its magnitude is comparable with that of a signal of similar nature from a SN. We show detection forecasts for these events, finding that they could be observable with the e-ASTROGAM telescope, thus opening a new window into both the astrophysics of these cataclysmic events, and of new particles beyond the standard model.