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There are two proposed explanations for ultraluminous X-ray sources (ULXs)with luminosities in excess of $10^{39}$ erg s$^{-1}$. They could beintermediate-mass black holes (more than 100-1,000, solar masses, $M_\odot$)radiating at sub-maximal (sub-Eddington) rates, as in Galactic black-hole X-raybinaries but with larger, cooler accretion disks. Alternatively, they could bestellar-mass black holes radiating at Eddington or super-Eddington rates. Onits discovery, M101 ULX-1 had a luminosity of $3\times10^{39}$ erg s$^{-1}$ anda supersoft thermal disk spectrum with an exceptionally low temperature --uncomplicated by photons energized by a corona of hot electrons -- moreconsistent with the expected appearance of an accreting intermediate-mass blackhole. Here we report optical spectroscopic monitoring of M101 ULX-1. We confirmthe previous suggestion that the system contains a Wolf-Rayet star, and revealthat the orbital period is 8.2 days. The black hole has a minimum mass of$5M_\odot$, and more probably a mass of $20-30M_\odot$, but we argue that it isvery unlikely to be an intermediate-mass black hole. Therefore itsexceptionally soft spectra at high Eddinton ratios violate the expectations foraccretion onto stellar-mass black holes. Accretion must occur from capturedstellar wind, which has hitherto been thought to be so inefficient that itcould not power an ultraluminous source.

Puzzling accretion onto a black hole in the ultraluminous X-ray source M101 ULX-1