Direct Detection of Warm Dark Matter in the X‐Ray

We point out a serendipitous link between warm dark matter (WDM) models forstructure formation on the one hand and the high sensitivity energy range (1-10keV) for x-ray photon detection on the Chandra and XMM-Newton observatories onthe other. This fortuitous match may provide either a direct detection of thedark matter or exclusion of many candidates. We estimate expected x-ray fluxesfrom field galaxies and clusters of galaxies if the dark matter halos of theseobjects are composed of WDM candidate particles with rest masses in thestructure formation-preferred range (~1 keV to ~20 keV) and with smallradiative decay branches. Existing observations lead us to conclude that forsinglet neutrinos (possessing a very small mixing with active neutrinos) to bea viable WDM candidate they must have rest masses < 5 keV in the zero leptonnumber production mode. Future deeper observations may detect or exclude theentire parameter range for the zero lepton number case, perhaps restricting theviability of singlet neutrino WDM models to those where singlet production isdriven by a significant lepton number. The Constellation X project has thecapability to detect/exclude singlet neutrino WDM for lepton number values upto 10% of the photon number. We also consider diffuse x-ray backgroundconstraints on these scenarios. These same x-ray observations additionally mayconstrain parameters of active neutrino and gravitino WDM candidates.Comment: 11 pages, 6 figures, replacement to match ApJ versio