  A dark photon is a well-motivated new particle which, as a component of an
associated dark sector, could explain dark matter. One strong limit on dark
photons arises from excessive cooling of supernovae. We point out that even at
couplings where too few dark photons are produced in supernovae to violate the
cooling bound, they can be observed directly through their decays. Supernovae
produce dark photons which decay to positrons, giving a signal in the 511 keV
annihilation line observed by SPI/INTEGRAL. Further, prompt gamma-ray emission
by these decaying dark photons gives a signal for gamma-ray telescopes.
Existing GRS observations of SN1987a already constrain this, and a future
nearby SN could provide a detection. Finally, dark photon decays from
extragalactic SN would produce a diffuse flux of gamma rays observable by
detectors such as SMM and HEAO-1. Together these observations can probe dark
photon couplings several orders of magnitude beyond current constraints for
masses of roughly 1 - 100 MeV.
