The X-ray source M33 X-7 in the nearby galaxy Messier 33 is among the most massive X-ray binary stellar systems known, hosting a rapidly spinning, 15.65M⊙ black hole orbiting an underluminous, 70M⊙ main-sequence companion in a slightly eccentric 3.45-day orbit1,2 (M⊙, solar mass). Although post-main-sequence mass transfer explains the masses and tight orbit3, it leaves unexplained the observed X-ray luminosity, the star’s underluminosity, the black hole’s spin and the orbital eccentricity. A common envelope phase1, or rotational mixing4, could explain the orbit, but the former would lead to a merger and the latter to an overluminous companion. A merger would also ensue if mass transfer to the black hole were invoked for its spin-up5. Here we report simulations of evolutionary tracks which reveal that if M33 X-7 started as a primary body of 85M⊙–99M⊙ and a secondary body of 28M⊙–32M⊙, in a 2.8–3.1-d orbit, its observed properties can be consistently explained. In this model, the main-sequence primary transfers part of its envelope to the secondary and loses the rest in a wind; it ends its life as a ∼16M⊙ helium star with an iron–nickel core that collapses to a black hole (with or without an accompanying supernova). The release of binding energy, and possibly collapse asymmetries, ‘kick’ the nascent black hole into an eccentric orbit. Wind accretion explains the X-ray luminosity, and the black-hole spin can be natal.
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We thank N. Ivanova, A. Heger, A. Cantrell and C. Bailyn for discussions during the development of this project.
The authors declare no competing financial interests.
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Valsecchi, F., Glebbeek, E., Farr, W. et al. Formation of the black-hole binary M33 X-7 through mass exchange in a tight massive system. Nature 468, 77–79 (2010). https://doi.org/10.1038/nature09463
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