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

There are two proposed explanations for ultraluminous X-ray sources (ULXs) with luminosities in excess of $10^{39}$ erg s$^{-1}$. They could be intermediate-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-ray binaries but with larger, cooler accretion disks. Alternatively, they could be stellar-mass black holes radiating at Eddington or super-Eddington rates. On its discovery, M101 ULX-1 had a luminosity of $3\times10^{39}$ erg s$^{-1}$ and a supersoft thermal disk spectrum with an exceptionally low temperature -- uncomplicated by photons energized by a corona of hot electrons -- more consistent with the expected appearance of an accreting intermediate-mass black hole. Here we report optical spectroscopic monitoring of M101 ULX-1. We confirm the previous suggestion that the system contains a Wolf-Rayet star, and reveal that 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 is very unlikely to be an intermediate-mass black hole. Therefore its exceptionally soft spectra at high Eddinton ratios violate the expectations for accretion onto stellar-mass black holes. Accretion must occur from captured stellar wind, which has hitherto been thought to be so inefficient that it could not power an ultraluminous source.