  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.
