Overview

• GW231123 was captured on November 23, 2023, during the collaboration’s fourth observing run and revealed two black holes of roughly 100 and 140 solar masses colliding.
• The resulting 225-solar-mass remnant surpasses the previous high-mass record and occupies a range excluded by standard stellar-evolution theories.
• Each progenitor black hole spins at 80–90% of the theoretical maximum, complicating waveform interpretation and indicating prior merger histories.
• Existing waveform models and observatories were tested to their limits by GW231123, prompting the development of more sophisticated modelling techniques.
• The LVK collaboration will continue its O4 observing run through November 2025 and aims to publish comprehensive analyses of early data later this summer.