Constraints on axion-like particles from GW170817

Since the 2017 observation of gravitational and electromagnetic signals from a neutron star merger, binary mergers have emerged as a powerful environment in which to study physics beyond the standard model (BSM). As an example, I consider axion-like particles (ALPs) that couple to photons, and are produced by photon fusion and the Primakoff process. When two neutron stars merge, if ALPs exist, they would be produced in copious amounts from the moment of merger throughout the lifetime of the merger remant. If their interations are sufficiently feeble, the ALPs escape the merger system and eventually decay into two photons, giving rise to a gamma-ray signal that can last for up to several hours. I discuss the constraints that can be placed on ALPs from Fermi-LAT's observation of GW170817.