Formation constraints indicate a black-hole accretor in 47 Tuc X9

TL;DR

This paper presents evidence that 47 Tuc X9 is a stellar-mass black hole accreting from a white dwarf, formed through dynamical interactions in a globular cluster, with implications for gravitational wave sources.

Contribution

It demonstrates that stable mass transfer in 47 Tuc X9 requires a black hole accretor and outlines a formation scenario involving dynamical collisions and common-envelope evolution.

Findings

Mass transfer stability implies a black hole accretor.

Formation involves collision with a giant star and common-envelope ejection.

The system's properties align with predicted formation rates in globular clusters.

Abstract

The luminous X-ray binary 47 Tuc X9 shows radio and X-ray emission consistent with a stellar-mass black hole accreting from a carbon-oxygen white dwarf. Its location, in the core of the massive globular cluster 47 Tuc, hints at a dynamical origin. We assess the stability of mass transfer from a carbon-oxygen white dwarf onto compact objects of various masses, and conclude that for mass transfer to proceed stably the accretor must, in fact, be a black hole. Such systems can form dynamically by the collision of a stellar-mass black hole with a giant star. Tidal dissipation of energy in the giant's envelope leads to a bound binary with a pericentre separation less than the radius of the giant. An episode of common-envelope evolution follows, which ejects the giant's envelope. We find that the most likely target is a horizontal-branch star, and that a realistic quantity of subsequent dynamical hardening is required for the resulting binary to merge via gravitational wave emission. Observing one binary like 47 Tuc X9 in the Milky Way globular cluster system is consistent with the expected formation rate. The observed 6.8-day periodicity in the X-ray emission may be driven by eccentricity induced in the UCXB's orbit by a perturbing companion.