Gamma rays from dark mediators in white dwarfs

We consider self-annihilation of dark matter, $chi$, into metastable mediators, $Y$, and their subsequent decay into photons inside white dwarfs. We focus on reactions of the type $chi bar{chi}rightarrow YY$, where mediators, besides having a finite decay lifetime at rest $tau_{rm rest}lesssim 1$ s, may suffer energy loss in the medium before they decay into photons, $Y rightarrow gammagamma$. We obtain attenuated gamma-ray luminosities arising from the combination of both effects. Using complementary sets of astrophysical measurements from cold white dwarfs in the M4 globular cluster as well as direct/indirect dark matter searches we discuss further constraints on dark mediator lifetimes.