  Though theoretically expected, the charge exchange emission from galaxy
clusters has not yet been confidently detected. Accumulating hints were
reported recently, including a rather marginal detection with the Hitomi data
of the Perseus cluster. As suggested in Gu et al. (2015), a detection of charge
exchange line emission from galaxy clusters would not only impact the
interpretation of the newly-discovered 3.5 keV line, but also open up a new
research topic on the interaction between hot and cold matter in clusters. We
aim to perform the most systematic search for the O VIII charge exchange line
in cluster spectra using the RGS on board XMM. We introduce a sample of 21
clusters observed with the RGS. The dominating thermal plasma emission is
modeled and subtracted with a two-temperature CIE component, and the residuals
are stacked for the line search. The systematic uncertainties in the fits are
quantified by refitting the spectra with a varying continuum and line
broadening. By the residual stacking, we do find a hint of a line-like feature
at 14.82 A, the characteristic wavelength expected for oxygen charge exchange.
This feature has a marginal significance of 2.8 sigma, and the average
equivalent width is 2.5E-4 keV. We further demonstrate that the putative
feature can be hardly affected by the systematic errors from continuum
modelling and instrumental effects, or the atomic uncertainties of the
neighbouring thermal lines. Assuming a realistic temperature and abundance
pattern, the physical model implied by the possible oxygen line agrees well
with the theoretical model proposed previously to explain the reported 3.5 keV
line. If the charge exchange source indeed exists, we would expect that the
oxygen abundance is potentially overestimated by 8-22% in previous X-ray
measurements which assumed pure thermal lines.
