Decaying Dark Matter and the X-Ray Line Budget: Lifetimes, Line Fluxes and Why a Single Unidentified Line Cannot Settle the Question

This study computes X-ray line fluxes from decaying dark matter, for candidate masses that place the line between 1 and 20 keV, and compares them with published lifetime limits. The line flux depends on the decay rate and on the column density of dark matter toward the target, not on the square of the density, so decay searches favour massive, nearby, dark-matter-rich systems viewed over wide fields. A lifetime of 1e28 seconds gives line surface brightnesses at the edge of current instrument sensitivity for the most favourable targets, and the well-known feature near 3.5 keV would require a lifetime in a range that other targets then test independently. Consistency across targets with very different column densities is the decisive test, and the paper tabulates the ratios that consistency requires. All results are computed, not measured.