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 $\sim$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 is modeled using both background-subtracted and
background-modeled approaches to account for the particle instrument
background, demonstrating statistical limitations of the currently-available
$\sim$1 Ms of particle background data. A non-detection is reported in the
total data set, allowing us to set an upper-limit on 3.5 keV line flux and
constrain the sterile neutrino dark matter mixing angle. The upper-limit on
sin$^2$(2$\theta$) is $2.58 \times 10^{-11}$ (though systematic uncertainty may
increase this by a factor of $\sim$2), corresponding to the upper-limit on 3.5
keV line flux of $2.34 \times 10^{-7}$ ph s$^{-1}$ cm$^{-2}$. These limits show
consistency with recent constraints and several prior detections.
Non-detections 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 non-detection of any unidentified emission lines
across our continuum to further constrain the sterile neutrino parameter space.
