  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 candidate. We aim at putting constraints on the proposed line
emission in a large sample of Chandra-observed clusters and obtain limits on
the mixing-angle in a 7.1 keV sterile neutrino dark matter scenario. For a
sample of 33 high-mass clusters of galaxies we merge all observations from the
Chandra data archive. Each cluster has more than 100 ks of combined exposure.
The resulting high signal-to-noise spectra are used to constrain the flux of an
unidentified line emission at 3.55 keV in the individual spectra and a merged
spectrum of all clusters. We obtained very detailed spectra around the 3.55 keV
range and limits on an unidentified emission line. Assuming all dark matter
were made of 7.1 keV sterile neutrinos the upper limits on the mixing angle are
$\rm{sin^2(2\Theta)}$ $\rm{<10.1\times10^{-11}}$ from ACIS-I, and
$\rm{<40.3\times10^{-11}}$ from ACIS-S data at 99.7 per cent confidence level.
We do not find evidence for an unidentified emission line at 3.55 keV. The
sample extends the list of objects searched for an emission line at 3.55 keV
and will help to identify the best targets for future studies of the potential
dark matter decay line with upcoming X-ray observatories like Hitomi (Astro-H),
eROSITA, and Athena.
