Puzzling accretion onto a black hole in the ultraluminous X-ray source M 101 ULX-1

• Published in: Nature (London) Vol. 503; no. 7477; pp. 500 - 503
• Main Authors: Liu, Ji-Feng, Bregman, Joel N., Bai, Yu, Justham, Stephen, Crowther, Paul
• Format: Journal Article
• Language: English
• Published: London Nature Publishing Group UK 29.11.2013; Nature Publishing Group
• Subjects: 639/33/34/867; Accretion; Accretion disks; Astrophysics; Black holes; Black holes (Astronomy); Helium; Humanities and Social Sciences; letter; Low temperature; Luminosity (Astronomy); multidisciplinary; Primaries & caucuses; Properties; Science; X-ray astronomy; X-rays
• ISSN: 0028-0836; 1476-4687
• DOI: 10.1038/nature12762

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. Accretion power for an ultraluminous X-ray source Ultraluminous X-ray sources (ULXs), more luminous than known stellar processes but less so than active galactic nuclei, are generally thought to be powered by either intermediate-mass black holes or smaller, stellar-mass black holes radiating at higher rates. Analysis of the optical spectra of M 101 ULX-1, a variable source in the nearby spiral galaxy M 101, suggests that matters may be more complicated. The luminosity of the source at the time of discovery was within the range expected for a ULX harbouring an intermediate-mass black hole. But the optical spectra reported here are consistent with the presence of a stellar-mass black hole, probably with a mass of 20–30 solar masses. The authors suggest that M 101 ULX-1 is driven by a black hole accreting gas from the wind of a companion star, a mechanism that had previously been rejected on the grounds that it was thought too slow for the purpose. There are two proposed explanations for ultraluminous X-ray sources 1 , 2 (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, ) radiating at sub-maximal (sub-Eddington) rates, as in Galactic black-hole X-ray binaries but with larger, cooler accretion disks 3 , 4 , 5 . Alternatively, they could be stellar-mass black holes radiating at Eddington or super-Eddington rates 2 , 6 . On its discovery, M 101 ULX-1 4 , 7 had a luminosity of 3 × 10 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 3 , 4 . Here we report optical spectroscopic monitoring of M 101 ULX-1. We confirm the previous suggestion 8 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 5 , and more probably a mass of 20 −30 , but we argue that it is very unlikely to be an intermediate-mass black hole. Therefore, its exceptionally soft spectra at high Eddington ratios violate the expectations for accretion onto stellar-mass black holes 9 , 10 , 11 . Accretion must occur from captured stellar wind, which has hitherto been thought to be so inefficient that it could not power an ultraluminous source 12 , 13 .