GW231123: A Binary Black Hole Merger with Total Mass 190–265 M⊙

On 2023 November 23 the two LIGO observatories both detected GW231123, a gravitational-wave signal consistent with the merger of two black holes with masses 137^{+23}_{-18}\, M_\odot and 101^{+22}_{-50}\, M_\odot (90\% credible intervals), at luminosity distance 0.7-4.1 Gpc and redshift of 0.40^{+0.27}_{-0.25}, and a network signal-to-noise ratio of \sim20.7. Both black holes exhibit high spins, 0.9^{+0.10}_{-0.19} and 0.80^{+0.20}_{-0.52} respectively. A massive black hole remnant is supported by an independent ringdown analysis. Some properties of GW231123 are subject to large systematic uncertainties, as indicated by differences in inferred parameters between signal models. The primary black hole lies within or above the theorized mass gap where black holes between 60-130 M_\odot should be rare due to pair instability mechanisms, while the secondary spans the gap. The observation of GW231123 therefore suggests the formation of black holes from channels beyond standard stellar collapse, and that intermediate-mass black holes of mass \sim200 M_\odot form through gravitational-wave driven mergers.