Context : 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 .

Aims : We aim to further constrain the line strength and its implied mixing angle under the assumption that all DM is made of sterile neutrinos .

Methods : 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 .

Results : We found tight constraints on the allowed flux for an additional line at 3.55 keV with a positive ( \mathrm { \sim 1.5 \sigma } ) best fit value \mathrm { F _ { X } ^ { 3.55 keV } \approx ( 4.5 \pm 3.5 ) \times 10 ^ { -7 } ~ { } cts~ { } cm ^ { -2 } ~ { } s ^ { % -1 } } .
This would translate into a mixing angle of \mathrm { sin ^ { 2 } ( 2 \Theta ) \approx ( 2.3 \pm 1.8 ) \times 10 ^ { -11 } } which , while consistent with some recent results , is in tension with earlier detections .

Conclusions : 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 .