GW231123

GW231123 was a gravitational wave detected by the two LIGO detectors on 23 November 2023. As of 2025, it is the largest binary black hole merger yet detected, at 225+26
−43 solar masses (M_☉) for the final object. One or both of the objects involved in the merger may have been in the black hole mass gap.^[1][2]

GW231123 was the result of the merger of a 137+22
−17 M_☉ black hole and a 103+20
−52 M_☉ black hole creating a new intermediate-mass black hole. It occurred at an estimated distance of 2.2+1.9
−1.5 Gpc. They have unusually large spins, with a magnitude of 0.9+0.10
−0.19 and 0.8+0.20
−0.51 for the two components respectively.^[3]

The high mass of both components of the binary black hole raises questions on its formation, as some very massive stars are thought to undergo pair-instability supernova, which do not leave a remnant; this is thought to result in a gap in the black hole mass distribution between 60 and 130 M_☉ (although there is some uncertainty on the limits). This suggests that the system has undergone a more complex process, e.g. with the black holes possibly resulting from earlier mergers, which is known as a "hierarchical" merger (this hypothesis is in particular supported by the high spins of the black holes).^[3]

As the signal is extremely short, it is also possible that the signal originates from another type of system, such as cosmic strings, although it is considered unlikely.^[3]