# 7.1 keV sterile neutrino dark matter constraints from a deep Chandra X-ray observation of the Galactic bulge Limiting Window

## TL;DR

This study uses deep X-ray observations of the Galactic bulge to constrain the presence of 7.1 keV sterile neutrino dark matter, providing tight limits on the mixing angle and discussing implications for future missions.

## Contribution

It presents new constraints on sterile neutrino dark matter using deep Chandra data and systematic uncertainty analysis, improving previous limits and guiding future observations.

## Key findings

- Tight constraints on the 3.55 keV line flux.
- Derived mixing angle limit of approximately 2.3×10⁻¹¹.
- Inner Milky Way identified as promising target for future DM searches.

## Abstract

Recently an unidentified emission line at 3.55 keV has been detected in X-ray spectra of clusters of galaxies. The line has been discussed as a possible decay signature of 7.1 keV sterile neutrinos, which have been proposed as a dark matter (DM) candidate. We aim to further constrain the line strength and its implied mixing angle under the assumption that all DM is made of sterile neutrinos. The X-ray observations of the Limiting Window (LW) towards the Galactic bulge (GB) offer a unique dataset for exploring DM lines. We characterize the systematic uncertainties of the observation and the fitted models with simulated X-ray spectra. In addition we discuss uncertainties of indirect DM column density constraints towards the GB to understand systematic uncertainties in the assumed DM mass in the field of view of the observation. We found tight constraints on the allowed flux for an additional line at 3.55 keV with a positive ($\mathrm{\sim1.5\sigma}$) best fit value $\mathrm{F_X^{3.55keV}\approx(4.5\pm3.5)\times10^{-7}~cts~cm^{-2}~s^{-1}}$. This would translate into a mixing angle of $\mathrm{sin^{2}(2\Theta)\approx(2.3\pm1.8)\times10^{-11}}$ which, while consistent with some recent results, is in tension with earlier detections. We used a very deep dataset with well understood systematics to derive tight constraints on the mixing angle of a 7.1 keV sterile neutrino DM. The results highlight that the inner Milky Way will be a good target for DM searches with upcoming missions like eROSITA, XRISM, and ATHENA.