Gamma-Ray Flashes from Dark Photons in Neutron Star Mergers
  In this letter we begin the study of visible dark sector signals coming from
binary neutron star mergers. We focus on dark photons emitted in the 10 ms - 1
s after the merger, and show how they can lead to bright transient gamma-ray
signals. The signal will be approximately isotropic, and for much of the
interesting parameter space will be close to thermal, with an apparent
temperature of about $100$ keV. These features can be used to distinguish the
dark photon signal from the expected short gamma-ray bursts produced in neutron
star mergers, which are beamed in a small angle and non-thermal. We calculate
the expected signal strength and show that for dark photon masses in the
$1-100$ MeV range it can easily lead to total luminosities larger than
$10^{46}$ ergs for much of the unconstrained parameter space. This signal can
be used to probe a large fraction of the unconstrained parameter space
motivated by freeze-in dark matter scenarios with interactions mediated by a
dark photon in that mass range. We also comment on future improvements when
proposed telescopes and mid-band gravitational detectors become operational.
