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Recently an unidentified emission line at 3.55 keV has been detected in X-rayspectra of clusters of galaxies. The line has been discussed as a possibledecay signature of 7.1 keV sterile neutrinos, which have been proposed as adark matter (DM) candidate. We aim to further constrain the line strength andits implied mixing angle under the assumption that all DM is made of sterileneutrinos. The X-ray observations of the Limiting Window (LW) towards theGalactic bulge (GB) offer a unique dataset for exploring DM lines. Wecharacterize the systematic uncertainties of the observation and the fittedmodels with simulated X-ray spectra. In addition we discuss uncertainties ofindirect DM column density constraints towards the GB to understand systematicuncertainties 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 3.55keV 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, whileconsistent with some recent results, is in tension with earlier detections. Weused a very deep dataset with well understood systematics to derive tightconstraints on the mixing angle of a 7.1 keV sterile neutrino DM. The resultshighlight that the inner Milky Way will be a good target for DM searches withupcoming missions like eROSITA, XRISM, and ATHENA.

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