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

TL;DR

This paper proposes a new evolutionary scenario for the formation of the massive black-hole binary M33 X-7, explaining its properties through initial mass transfer, wind loss, and natal black hole spin, resolving previous inconsistencies.

Contribution

It introduces a detailed evolutionary model starting from specific initial masses and orbit, successfully explaining M33 X-7's observed properties and black hole spin origin.

Findings

Initial primary mass: 85-99 Msun, secondary: 28-32 Msun, orbit: 2.8-3.1 days.

Mass transfer and wind loss lead to a ~16 Msun He star collapsing into a black hole.

Wind accretion accounts for X-ray luminosity; black hole spin is natal.

Abstract

M33 X-7 is among the most massive X-Ray binary stellar systems known, hosting a rapidly spinning 15.65 Msun black hole orbiting an underluminous 70 Msun Main Sequence companion in a slightly eccentric 3.45 day orbit. Although post-main-sequence mass transfer explains the masses and tight orbit, it leaves unexplained the observed X-Ray luminosity, star's underluminosity, black hole's spin, and eccentricity. A common envelope phase, or rotational mixing, 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. Here we report that, if M33 X-7 started as a primary of 85-99 Msun and a secondary of 28-32 Msun, in a 2.8-3.1 day orbit, its observed properties can be consistently explained. In this model, the Main Sequence primary transferred part of its envelope to the secondary and lost the rest in a wind; it ended its life as a ~16 Msun He star with a Fe-Ni core which collapsed to a black hole (with or without an accompanying supernova). The release of binding energy and, possibly, collapse asymmetries "kicked" the nascent black hole into an eccentric orbit. Wind accretion explains the X-Ray luminosity, while the black hole spin can be natal.