Shedding stray light on decaying light dark matter: Constraints from NuSTAR x-ray observations

Light dark matter (DM) [ mass ≲ O ( 100 )     keV ] remains challenging to detect in several ongoing indirect detection experiments due to threshold limitations. Recent observations of diffuse x-ray photons from the NuSTAR stray-light (SL) data provide a powerful avenue to probe such light DM through its decay signatures in the galactic halo. This work explores the indirect detection prospects of decaying electrophilic scalar DM, electrophilic and photophilic axionlike particle DM, and dark photon DM using the recent NuSTAR SL data. We find that for DM scenarios producing monochromatic two-photon signals, NuSTAR SL data can yield the strongest indirect detection bound in the 7–36 keV mass range. In contrast, for dark photon (vector) DM featuring a continuous three-photon spectrum, the strongest indirect detection upper bound arises in the 22–65 keV mass range. Additionally, we discuss the detection prospects of inelastic DM where the heavier DM decays to a two or three-photon final state along with a massive lighter dark sector particle. By comparing the resulting continuous photon spectra with the NuSTAR SL data, we obtain the most stringent lower bound on the lifetime of such DM for the mass splitting Δ m in the range 3 keV–100 keV.