Understanding super-Eddington accretion onto Ultraluminous X-ray sources through Integral-Field Unit spectroscopy

The presence of extended (50--400 pc) large ionized nebulae surrounding Ultraluminous X-ray sources (ULXs) result from copious amounts of radiation and powerful radiatively-driven outflows produced in super-Eddington accretion flows. Observing such interaction between the central ULX and the environment offers an opportunity to understand super-Eddington accretion feedback and constrain its accretion flow geometry. In this talk, I will present ULXs as laboratories for super-Eddington accretion and show how using Integral Field Unit spectroscopy allows us to probe the interaction between the ULX and its environment in unprecedented level of detail. I will focus on the discovery of a superbubble (~400pc) around the archetypal ULX NGC1313X-1 with MUSE-IFU, showing how the ULX is powering both a shock-ionised bubble and an extended X-ray excited region. I will show how thanks to the spatially-resolved information offered by MUSE-IFU, we can estimate the kinematic power of the super-Eddington winds and how, coupling the X-ray ionized nebular emission with multi-band spectroscopy and Cloudy modelling, we are also able to constrain the anisotropy of the emission. I will finally discuss whether our observations support super-Eddington accretion models which invoke beamed emission.