Overview

• The GW231123 signal was officially announced at the GR-Amaldi conference in Glasgow on July 14, confirming a black hole remnant of about 225 solar masses.
• Analysis shows two progenitor black holes of roughly 103 and 137 solar masses spun at near-relativistic speeds and merged in under 0.1 seconds.
• The 225-solar-mass remnant falls squarely within the pair-instability mass gap, implying it could not have formed through direct stellar collapse.
• Exceptional spin rates and the fleeting ringdown phase have driven efforts to refine waveform models capable of interpreting similarly brief, high-spin signals.
• Researchers are now combing through O4 data to identify more extreme events and to test hierarchical-merger scenarios for the growth of intermediate-mass black holes.