Recycling serendipitous observations of XRISM for dark matter searches

Abstract: Searches for radiative decays of keV-mass dark matter have been limited by energy resolution. We propose a strategy for the XRISM/Resolve low count rate observations, and search the 3.48-3.56 keV band for a decay line from the Milky Way halo. Despite a lower exposure, our bounds are stronger than those from XRISM observations of galaxy clusters. We also identify a residual at E = 3.5311 keV with ~3sigma local significance, coincident with the previously reported unidentified feature, and having no plausible atomic or instrumental counterpart. These bounds will continue to tighten as additional pointings become available.