The ultraluminous X-ray source M 101 ULX-1 consists of a black hole orbiting a Wolf-Rayet star; optical spectroscopy now shows that the orbital period is 8.2 days, suggesting that the black hole has a mass in the range 5 to 30 solar masses, though the X-ray spectra are unlike what is expected from accretion onto a stellar-mass black hole—accretion must occur from captured stellar wind, which has hitherto been thought to be so inefficient that it could not power an ultraluminous source.
- Ji-Feng Liu
- Joel N. Bregman
- Paul Crowther