086481

In recent work done with XMM-Newton comma we have discovered that ultraluminous X-ray sources have strong relativistic winds in agreement with theoretical predictions of super-Eddington accretion. Such winds vary on timescales of days and can become optically thick enough to block and reprocess the disc X-ray photons comma thus changing the spectral shape of the source into an ultraluminous supersoft X-ray source. We propose to test this hypothesis comma by studying a transitional object where the wind is so strong that it is already causing significant absorption of the harder X-ray continuumdoublePoint NGC 55 ULX. A 130ks XMM observation triggered during a high X-ray flux state will enable to perform time--flux-resolved X-ray spectroscopy of the wind and probe the transition from supersoft to soft ULX.