Neutrino lines and photon continua from cascade dark matter decay

We investigate the two-body decay of fermionic dark matter, $χ(\barχ)\to X+ν(\barν)$, where the light mediator $X$ subsequently decays into photons. We consider two benchmark models: an axion-like particle with $a\toγγ$, and a kinetically mixed dark vector with $A'\to3γ$. This decay topology produce