  In recent work with high-resolution grating spectrometers (RGS) aboard
XMM-Newton Pinto et al. (2016) have discovered that two bright and archetypal
ultraluminous X-ray sources (ULXs) have strong relativistic winds in agreement
with theoretical predictions of high accretion rates. It has been proposed that
such winds can become optically thick enough to block and reprocess the disk
X-ray photons almost entirely, making the source appear as a soft thermal
emitter or ultraluminous supersoft X-ray source (ULS). To test this hypothesis
we have studied a ULX where the wind is strong enough to cause significant
absorption of the hard X-ray continuum: NGC 55 ULX. The RGS spectrum of NGC 55
ULX shows a wealth of emission and absorption lines blueshifted by significant
fractions of the light speed (0.01 - 0.20)c indicating the presence of a
powerful wind. The wind has a complex dynamical structure with the ionization
state increasing with the outflow velocity, which may indicate launching from
different regions of the accretion disk. The comparison with other ULXs such as
NGC 1313 X-1 and NGC 5408 X-1 suggests that NGC 55 ULX is being observed at
higher inclination. The wind partly absorbs the source flux above 1 keV,
generating a spectral drop similar to that observed in ULSs. The softening of
the spectrum at lower (~ Eddington) luminosities and the detection of a soft
lag agree with the scenario of wind clumps crossing the line of sight, partly
absorbing and reprocessing the hard X-rays from the innermost region.
