Formation of the black-hole binary M33 X-7 through mass exchange in a tight massive system

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(circle dot) black hole orbiting an underluminous, 70M(circle dot) main-sequence companion in a slightly eccentric 3.45-day orbit(1,2) (M-circle dot, solar mass). Although post-main-sequence mass transfer explains the masses and tight orbit(3), it leaves unexplained the observed X-ray luminosity, the star's underluminosity, the black hole's spin and the orbital eccentricity. A common envelope phase(1), or rotational mixing(4), 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-up(5). Here we report simulations of evolutionary tracks which reveal that if M33 X-7 started as a primary body of 85M(circle dot)-99M(circle dot) and a secondary body of 28M(circle dot)-32M(circle dot), 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(circle dot) 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.