Gravitational Wave Evidence of Spin Energy Extraction from Black Holes

Relativistic jets have been found for decades as a key phenomenon in active galactic nuclei (AGNs), compact binary systems, and gamma-ray bursts (GRBs), yet their energy resources remain a mystery. Two competing ideas prevail: one attributes jet energy to accretion power of the black hole (BH), the other, more interestingly, to magnetic extraction of rotational energy from the BH. A decisive observational distinction between them is still elusive. We propose that BHs remnant from their natal GRB activity can serve as a critical testbed to discriminate between these two scenarios. Via analytical approaches, we demonstrate that extraction of rotational energy to power jets during the GRB phase drives the remnant BH to a universal equilibrium spin, independent of accretion history, initial spin, and mass. This model predicts a stellar-mass BH population with this universal spin, a hallmark of BH spin energy extraction. Testing against the 4th gravitational wave (GW) catalogue (GWTC-4.0), we find a statistically robust dominant population where secondary BH spins are narrowly centered at $\sim0.05$. These findings provide strong new evidence for BH spin energy extraction.