“…Space-based x-ray observatories such as HEAO-1 [59], Chandra [60,61], XMM-Newton [59,[62][63][64], Suzaku [39,65], Fermi-GBM [40], and INTEGRAL [41,66] have provided the most robust constraints on the χ → ν þ γ decay rate for m χ ≃ 1-100 keV. The observation of an unknown x-ray line at E ≃ 3.5 keV ("the 3.5-keV line") in several analyses [34][35][36] has led to much interest, as well as many follow-up analyses using different instruments and astrophysical targets [30,38,39,63,64,[67][68][69][70][71][72][73][74][75][76][77][78][79][80][81][82][83]. Some suggest that the 3.5-keV line may be a signature of sterileneutrino DM [84] or other DM candidates [85][86][87][88][89]; alternatively, modeling systematics [69,71] or novel astrophysical processes [90,<...>…”