Gamma-ray line constraints on coy dark matter

Coy dark matter is an effective scheme in which a fermionic dark matter candidate interacts with the standard model fermions via a pseudoscalar mediator. This simple setup avoids the strong constraints posed by direct detection experiments in a natural way and explains, on top of the observed dark matter relic abundance, the spatially extended γ -ray excess recently detected at the Galactic Center. In this paper we study the phenomenology of coy dark matter accounting for a novel signature of the model: the diphoton annihilation signal induced by the standard model fermions at the loop level. By challenging the model with the observations of spheroidal dwarf satellite galaxies and the results of γ -ray line searches obtained by the Fermi LAT experiment, we assess its compatibility with the measured dark matter relic abundance and the Galactic Center excesses. We show that despite the γ -ray line constraint rules out a significant fraction of the considered parameter space, the region connected to the observed Galactic Center excess remains currently viable. Nevertheless, we find that next-generation experiments such as DAMPE, HERD and GAMMA-400 have the potential to probe exhaustively this elusive scenario.