Abstract
The nature of extra-nuclear ultraluminous X-ray sources (ULX) in nearby galaxies continues to be an enigma, since their adopted isotropic high-energy output would surpass the Eddington limit of even the most massive stellar black holes. Many ultraluminous X-ray sources are surrounded by emission nebulae that show indications of both shock ionization and X-ray ionization. Relatively compact X-ray ionized nebulae can be used to independently infer the luminosities, and thus to exclude possible beaming effects into our line of sight. Larger bubble-like nebulae reach several hundred parsec diameters and provide important information on the formation and/or mass loss history of ULX. We point out the close relationship to microquasars and the previously unique SS 433 system with its radio nebula W 50.
