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 ( \sim 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 .