A detached stellar-mass black hole candidate in the globular cluster NGC 3201

TL;DR

This study reports the discovery of a candidate stellar-mass black hole in a globular cluster, identified through radial velocity variations indicating a binary system with an unseen black hole companion, providing new insights into black hole populations in clusters.

Contribution

First detection of a detached stellar-mass black hole in a globular cluster using spectroscopic observations and orbital analysis.

Findings

Black hole candidate has a minimum mass of 4.36 solar masses.

Radial velocity variations of about 100 km/s indicate a binary system.

The discovery constrains models of black hole formation and evolution in globular clusters.

Abstract

As part of our massive spectroscopic survey of 25 Galactic globular clusters with MUSE, we performed multiple epoch observations of NGC 3201 with the aim of constraining the binary fraction. In this cluster, we found one curious star at the main-sequence turn-off with radial velocity variations of the order of 100 km/s, indicating the membership to a binary system with an unseen component since no other variations appear in the spectra. Using an adapted variant of the generalized Lomb-Scargle periodogram, we could calculate the orbital parameters and found the companion to be a detached stellar-mass black hole with a minimum mass of 4.36 $\pm$ 0.41 solar masses. The result is an important constraint for binary and black hole evolution models in globular clusters as well as in the context of gravitational wave sources.