In this talk we discuss the constraints on the decaying sterile neutrinos obtained from NuSTAR and ART-XC. To date, the constraints obtained from 11 years of NuSTAR exposure are among the strongest constraints on the decaying sterile neutrinos presented in the literature. Additionally, we will demonstrate the constraints for the case of dark matter particles annihilation and show that in the energy range of a few keV, the constraints that can be obtained from NuSTAR and ART-XC are among the best in this energy range