Ultra-luminous X-ray sources: extreme accretion and feedback

Ultra-luminous X-ray sources (ULXs) are the most extreme members of the X-ray binary population, exhibiting X-ray luminosities that can surpass the 10^39 erg/s threshold (by orders of magnitude). They are mainly seen in external galaxies and are most preferentially found in star-forming galaxies with lower metallicities. The vast majority of these systems are now understood to be powered by super-Eddington accretion of matter onto stellar-mass compact objects (black holes and neutron stars). This is driven by the discovery of coherent pulsations, cyclotron lines and powerful winds in members of the ULX population. The latter was possible thanks to high-resolution X-ray spectrometers such as those aboard XMM-Newton. ULX winds carry a huge amount of power owing to their relativistic speeds (0.1-0.3 c) and are likely responsible for the ~100 pc superbubbles observed around many ULXs. The winds also regulate the amount of matter that can reach the central accretor. Their study is, therefore, essential to understanding how quickly compact objects can grow and how strong their feedback onto the surrounding medium can be. This may also be relevant to understand supermassive black hole growth, particularly in the early Universe. Here we provide an overview on ULX phenomenology, highlight some recent exciting results, and show how future missions such as XRISM and ATHENA will drive further significant progress in this field.