We point out a serendipitous link between warm dark matter (WDM) models for structure formation on the one hand and the high sensitivity energy range (1-10 keV) for x-ray photon detection on the Chandra and XMM-Newton observatories on the other. This fortuitous match may provide either a direct detection of the dark matter or exclusion of many candidates. We estimate expected x-ray fluxes from field galaxies and clusters of galaxies if the dark matter halos of these objects are composed of WDM candidate particles with rest masses in the structure formation-preferred range (~1 keV to ~20 keV) and with small radiative decay branches. Existing observations lead us to conclude that for singlet neutrinos (possessing a very small mixing with active neutrinos) to be a viable WDM candidate they must have rest masses < 5 keV in the zero lepton number production mode. Future deeper observations may detect or exclude the entire parameter range for the zero lepton number case, perhaps restricting the viability of singlet neutrino WDM models to those where singlet production is driven by a significant lepton number. The Constellation X project has the capability to detect/exclude singlet neutrino WDM for lepton number values up to 10% of the photon number. We also consider diffuse x-ray background constraints on these scenarios. These same x-ray observations additionally may constrain parameters of active neutrino and gravitino WDM candidates.
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