A Long Period Stellar-Mass Black Hole Binary in omega Centauri

arXiv:2606.18350 · astro-ph.GA, astro-ph.SR · Submitted 2026-06-16 · Read on arXiv

Matthew Whitaker, Evan Kerr, Anil Seth, Maximilian Häberle, Jay Strader, Jay Anderson, Andrea Bellini, Callie Clontz, Zack Freeman, Massimo Griggio, Sebastian Kamann, Mattia Libralato, Nadine Neumayer, Elena González Prieto, Carl L. Rodriguez, Sara Saracino, Peter Smith, Glenn van de Ven, Zixian Wang

University of Utah · University of Utah · University of Utah · European Southern Observatory · Michigan State University · Space Telescope Science Institute · Space Telescope Science Institute · Max-Planck-Institut für Astronomie · University of Utah · Space Telescope Science Institute · Liverpool John Moores University · INAF-Padova · Max-Planck-Institut für Astronomie · Center for Interdisciplinary Exploration & Research in Astrophysics · University of North Carolina at Chapel Hill · Osservatorio Astrofisico di Arcetri · Max-Planck-Institut für Astronomie · University of Vienna · University of Utah

astro-ph.GA, astro-ph.SR

Submitted: 2026-06-16

Comments: 23 pages, 9 figures; accepted for publication in the Astrophysical Journal Letters

License: http://creativecommons.org/licenses/by/4.0/

The gist: Modern simulations of stellar dynamics in globular clusters peg a dominant role for stellar-mass black holes, but direct evidence for black holes in clusters remains limited.

Terminology

Abstract

Modern simulations of stellar dynamics in globular clusters peg a dominant role for stellar-mass black holes, but direct evidence for black holes in clusters remains limited. We present the discovery of an astrometric stellar-mass black hole--main sequence star binary in omega Centauri, the most massive Galactic globular cluster, using Hubble Space Telescope data from the oMEGACat project and additional JWST data that span a total of 23 years. The luminous companion to the black hole is a main-sequence turnoff star, and has a period of 94+63-42 years, a semi-major axis of 31+15-12 AU, and an eccentricity of e=0.72+0.08-0.13. Since we observe the binary during periastron, the mass of the black hole is well-constrained even though we only observe a partial orbit: the inferred black hole mass is 4.46+1.22-1.01 M. We call this black hole oMEGACat BH-2. This is the first astrometric discovery of a stellar-mass black hole in a globular cluster, and is the longest period black hole binary system yet discovered. The low mass of this black hole is perhaps surprising given the low metallicity of the cluster, and shows that at least some low-mass black holes form at metallicity Z<10-3. We find that the binary is almost certainly dynamically formed and is soft, with an expected binary disruption timescale of about 800 Myr. While the total number of black hole binaries in omega Centauri is uncertain, we show that existing surveys only cover a small area of parameter space, and that the presence of additional detectable black hole binaries is likely.

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