Understanding state transitions in ULXs: wind spectroscopy of ULX bright states

In recent work done with XMM-Newton, we have discovered that ultraluminousX-ray sources (ULXs) have powerful winds in agreement with theoreticalpredictions of super-Eddington accretion. Such winds vary on shorttimescales and can temporarily block and reprocess the disc X-ray photons,thus changing the spectral shape of the source. Enhanced X-ray variabilityis seen during ULX highest states including flux dips in soft ULXs andflares in hard ones, possibly due to a variable wind in our line ofsight. We propose to test this, by studying the hard NGC 4559 X-7 andthe soft NGC 55 ULX-1. 130ks XMM-Newton observation per object triggeredduring a high state will enable us to perform time-/flux-resolved X-rayspectroscopy and probe the transition between soft, intermediate, andhard states.