The intriguing evolutionary history of the massive black hole X-ray binary M33 X-7

TL;DR

This paper presents a comprehensive evolutionary model for the massive black hole X-ray binary M33 X-7, explaining its unique properties through a detailed stellar evolution scenario consistent with all observations.

Contribution

The study introduces the first evolutionary scenario that accounts for all observed features of M33 X-7, including its high masses, tight orbit, and underluminous companion.

Findings

Proposed initial binary configuration with 85-99 Msun primary and 28-32 Msun companion.

Mass transfer and stellar wind processes explain current black hole and companion star masses.

Model aligns with observed orbital period and underluminosity of the companion star.

Abstract

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 Msun BH orbiting a 70 Msun 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 Msun primary and a 28-32 Msun companion in a Keplerian orbit of 2.8-3.1 days. In order to form a BH of 15.65 Msun, 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 Msun star.