Black hole ( BH ) X-ray binaries ( XRBs ) are X-ray luminous binary systems comprising a BH accreting matter from a companion star .
Understanding their origins sheds light on the still not well understood physics of BH formation .
M33 X-7 hosts one of the most massive stellar-mass BH among all XRBs known to date , a 15.65 M _ { \odot } BH orbiting a 70 M _ { \odot } companion star in a 3.45 day orbit .
The high masses of the two components and the tight orbit relative to the large H-rich stellar component challenge our understanding of the typically invoked BH-XRBs formation channels .
The measured underluminosity of the optical component further complicates the picture .
A solution to the evolutionary history of this system that can account for all its observed properties has yet to be presented , and here we propose the first scenario that is consistent with the complete set of current observational constraints .
In our model , M33 X-7 started its life hosting a 85-99 M _ { \odot } primary and a 28-32 M _ { \odot } companion in a Keplerian orbit of 2.8-3.1 days .
In order to form a BH of 15.65 M _ { \odot } , the initially most massive component transferred part of its envelope to the companion star and lost the rest in a strong stellar wind .
During this dynamically stable mass transfer phase the companion accreted matter , to become the presently underluminous 70 M _ { \odot } star .