General-relativistic Precession in a Black-Hole Binary

The general-relativistic phenomenon of spin-induced orbital precession has not yet been observed in strong-field gravity. Gravitational-wave observations of binary black holes (BBHs) are prime candidates, as we expect the astrophysical binary population to contain precessing binaries(1,2). Imprints of precession have been investigated in several signals(3-5), but no definitive identification of orbital precession has been reported in any of the 84 BBH observations so far(5-7) by the Advanced LIGO and Virgo detectors(8,9). Here we report the measurement of strong-field precession in the LIGO-Virgo-Kagra gravitational-wave signal GW200129. The binary's orbit precesses at a rate ten orders of magnitude faster than previous weak-field measurements from binary pulsars(10-13). We also find that the primary black hole is probably highly spinning. According to current binary population estimates, a GW200129-like signal is extremely unlikely, and therefore presents a direct challenge to many current binary-formation models.