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

TL;DR

This study used extensive Chandra observations to search for the 3.5 keV line associated with dark matter decay in the Milky Way, setting upper limits that constrain sterile neutrino dark matter models.

Contribution

It provides the most comprehensive non-detection analysis of the 3.5 keV line using archival data, constraining dark matter decay parameters and spatial distribution.

Findings

No significant 3.5 keV line detected across all data subsets.

Upper limits on sterile neutrino mixing angle are established.

Constraints are consistent with prior studies but do not exclude dark matter decay hypotheses.

Abstract

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.