  Most Ultraluminous X-ray sources (ULXs) are thought to be powered by
super-Eddington accretion onto stellar-mass compact objects. Accretors in this
extreme regime are naturally expected to ionise copious amounts of plasma in
their vicinity and launch powerful radiation-driven outflows from their discs.
High spectral resolution X-ray observations (with RGS gratings onboard
XMM-Newton) of a few ULXs with the best datasets indeed found complex line
spectra and confirmed such extreme (0.1-0.3c) winds. However, a search for
plasma signatures in a large ULX sample with a rigorous technique has never
been performed, thereby preventing us from understanding their statistical
properties such as the rate of occurrence, to constrain the outflow geometry
and its duty cycle. We developed a fast method for automated line detection in
X-ray spectra and applied it to the full RGS ULX archive, rigorously
quantifying the statistical significance of any candidate lines. Collecting the
135 most significant features detected in 89 observations of 19 objects, we
created the first catalogue of spectral lines detected in soft X-ray ULX
spectra. We found that the detected emission lines are concentrated around
known rest-frame elemental transitions and thus originate from low-velocity
material. The absorption lines instead avoid these transitions, suggesting they
were imprinted by blueshifted outflows. Such winds therefore appear common
among the ULX population. Additionally, we found that spectrally hard ULXs show
fewer line detections than soft ULXs, indicating some difference in their
accretion geometry and orientation, possibly causing over-ionisation of plasma
by the harder spectral energy distributions of harder ULXs.
