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(-18)(+23)M(circle dot) and 101(-50)(+22)M(circle dot) (90% credible intervals), at a luminosity distance of 0.7-4.1 Gpc, a redshift of 0.40+ 0.250.27, and with a network signal-to-noise ratio of similar to 20.7. Both black holes exhibit high spins-0.90(-0.19)(+)0.10 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 the inferred parameters between signal models. The primary black hole lies within or above the theorized mass gap where black holes between 60-130M(circle dot) 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 similar to 200M(circle dot) form through gravitational-wave-driven mergers.