XMM-Newton multi-year campaign on NGC 55 ULX-1

Context. Winds are an important ingredient in the evolution of X-ray binary (XRB) systems, particularly those at high accretion rates such as ultra-luminous X-ray sources (ULXs), because they may regulate the accretion of matter onto the compact object. Aims. Our aim is to understand the properties of ULX winds and their link with the source spectral and temporal behaviour. Methods. We performed high-resolution X-ray spectroscopy of the variable source NGC 55 ULX-1 to resolve emission and absorption lines as observed with XMM-Newton at different epochs. Optically thin plasma models were used to characterise the wind. Results. We confirmed and thoroughly strengthened previous evidence of outflows in NGC 55 ULX-1. The presence of radiative recombination signatures and the ratios of the fluxes of the emission lines favours photoionisation balance and low to moderate densities, which confirm that the lines originate from classical XRB disc winds. An in-depth parameter space exploration shows line emission from a slowly moving, cool, and variable plasma perhaps associated with a thermal wind. Mildly relativistic Doppler shifts (about −0.15c) associated with the absorption lines confirm, at higher confidence, the presence of powerful radiatively driven winds. Conclusions. The comparison between the results obtained at different epochs revealed that the wind responds to the variability of the underlying continuum. These variations may be used to understand the actual accretion regime and the nature of the source.