Formation of black-hole X-ray binaries in globular clusters

TL;DR

This paper investigates how black-hole white dwarf X-ray binaries form in globular clusters, emphasizing the role of triple interactions and the Kozai mechanism, and compares theoretical rates with observations.

Contribution

It introduces a detailed analysis of formation channels for BH-WD binaries, highlighting the importance of triple-induced mass transfer and Kozai cycles.

Findings

Triple interactions are crucial for BH-WD binary formation.

Kozai mechanism facilitates mass transfer in these binaries.

Formation rates are limited by black hole evaporation and cluster dynamics.

Abstract

Inspired by the recent identification of the first candidate BH-WD X-ray binaries, where the compact accretors may be stellar-mass black hole candidates in extragalactic globular clusters, we explore how such binaries could be formed in a dynamical environment. We provide analyses of the formation rates via well known formation channels like binary exchange and physical collisions and propose that the only possibility to form BH-WD binaries is via coupling these usual formation channels with subsequent hardening and/or triple formation. Indeed, we find that the most important mechanism to make a BH-WD X-ray binary from an initially dynamically formed BH-WD binary is triple induced mass transfer via the Kozai mechanism. Even using the most optimistic estimates for the formation rates, we cannot match the observationally inferred production rates if black holes undergo significant evaporation from the cluster or form a completely detached subcluster of black holes. We estimate that at least 1% of all formed black holes, or presumably 10% of the black holes present in the core now, must be involved in interactions with the rest of the core stellar population.