An Evolutionary Model for the Massive Black Hole X-Ray Binary M33 X-7

Black hole (BH) X-ray binaries (XRBs) are X-ray bright binary systems comprising a BH accreting matter from a companion star. Unravelling the evolutionary history of observed XRBs can shed light on how BHs form, evolve and interact with their host environment, processes that yet lack a conclusive physical explanation. Our focus here lies on M33 X-7, which is among the heaviest BH XRBs known to date, hosting a 15.65 M-circle dot BH orbiting a 70 M-circle dot companion star in a 3.45 day orbit. The high masses of the binary components and the tight orbit present a challenge for typically invoked BH-XRBs formation channels. The picture is further complicated by the measured underluminosity of the stellar component. Here we propose the first evolutionary scenario for M33 X-7 that is consistent with the complete set of current observational constraints. According to our model, the progenitors of M33 X-7 are constrained to host primaries and secondaries between 85-99 M-circle dot and 28-32 M-circle dot, respectively, and initial orbital periods between 2.8-3.1 days. In order to form a BH of 15.65 M-circle dot, the BH progenitor 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-circle dot star.