  Axion-like particles (ALP) can in principle be produced in very hot and dense
astrophysical environments, escape from the extreme object where such
conditions are met, and then be converted in gamma--rays in the magnetic fields
intervening between the event and the Earth. This process potentially offers a
new window on both the physics of the axions, and the inner working of the
astrophysical objects where they are produced. Interestingly, while this
process has been studied for core--collapse supernovae and other extreme
astrophysical events, no estimate exists for Neutron Star Mergers, objects
recently identified through the detection of gravitational waves. In this work
we study the production of ALPs in neutron star mergers, finding that for a
large region of the ALP parameter space its magnitude at the source is such to
produce a sizable gamma-ray signal at Earth. We show detection forecasts for
such events placed in nearby galaxies, finding that they are potentially
observable with the Fermi--LAT, thus opening a new window into both the
astrophysics of these cataclysmic events, and of new particles beyond the
standard model.
