Limits on the radiative decay of sterile neutrino dark matter from the unresolved cosmic and soft x-ray backgrounds

We present upper limits on line emission in the Cosmic X-ray background (CXB) that would be produced by decay of sterile neutrino dark matter. We employ the spectra of the unresolved component of the CXB in the Chandra Deep Fields North and South obtained with the Chandra CCD detector in the $E=0.8–9\mathrm{keV}$ band. The expected decay flux comes from the dark matter on the lines of sight through the Milky Way galactic halo. Our constraints on the sterile neutrino decay rate are sensitive to the modeling of the Milky Way halo. The highest halo mass estimates provide a limit on the sterile neutrino mass of $m_s<2.9\mathrm{keV}$ in the Dodelson-Widrow production model, while the lowest halo mass estimates provide the conservative limit of $m_s<5.7\mathrm{keV}$ ($2\sigma$). We also discuss constraints from a short observation of the softer ($E<1\mathrm{keV}$) X-ray background with a rocket-borne calorimeter by McCammon and collaborators.