Probing the Milky Way's Dark Matter Halo for the 3.5 keV Line

We present a comprehensive search for the 3.5 keV line using ∼51 Ms of archival Chandra observations peering through the Milky Way's dark matter halo from across the entirety of the sky, gathered via the Chandra Source Catalog Release 2.0. We consider the data's radial distribution, organizing observations into four data subsets based on angular distance from the Galactic center. All data are modeled using both background-subtracted and background-modeled approaches to account for the particle instrument background, demonstrating the statistical limitations of the currently available ∼1 Ms of particle background data. A nondetection is reported in the total data set, allowing us to set an upper limit on 3.5 keV line flux and constrain the sterile neutrino dark matter mixing angle. The upper limit on sin²(2θ) is $2.58\times {10}^{-11}$ (though systematic uncertainty may increase this by a factor of ∼2), corresponding to an upper limit on 3.5 keV line flux of $2.34\times {10}^{-7}$ photons s^-1 cm^-2. These limits show consistency with recent constraints and several prior detections. Nondetections are reported in all radial data subsets, allowing us to constrain the spatial profile of 3.5 keV line intensity, which does not conclusively differ from Navarro-Frenk-White predictions. Thus, while offering heavy constraints, we do not entirely rule out the sterile neutrino dark matter scenario or the more general decaying dark matter hypothesis for the 3.5 keV line. We have also used the nondetection of any unidentified emission lines across our continuum to further constrain the sterile neutrino parameter space.