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Ultraluminous X‐ray sources ( ULXs ) are bright extragalactic sources with X‐ray luminosities above 10 39  erg s −1  powered by accretion onto compact objects. According to the first studies performed with  XMM ‐Newton,  ULXs  seemed to be excellent candidates to host intermediate‐mass black holes (10 2 − 4  M  ⊙ ). However, in the last years, the interpretation of super‐Eddington accretion onto stellar‐mass black holes or neutron stars for most  ULXs  has gained a strong consensus. One critical missing piece to confirm the super‐Eddington scenario was the direct detection of the massive, radiatively driven winds expected as atomic emission/absorption lines in the  ULX  spectra. The first evidence for winds was found as residuals in the soft X‐ray spectra of  ULXs . Very recently, we have been able to resolve these residuals into rest‐frame emission and blueshifted (∼ 0.2 c ) absorption lines arising from highly ionized gas in the deep, high‐resolution  XMM ‐Newton spectra of two ULXs. The compact object is therefore surrounded by powerful ultrafast winds as predicted by models of hyper‐Eddington accretion. Here we discuss the relevance of these discoveries and the importance of further deep  XMM ‐Newton observations of powerful winds in many other ULXs to estimate the energetics of the wind, the geometry of the system, and the masses of the central accretors.

Ultrafast outflows in ultraluminous X‐ray sources