GW250114 reveals black hole horizon signatures

The event horizon of a black hole is characterized by two fundamental quantities: its rotation frequency and surface gravity, which govern the near-horizon frame-dragging and gravitational redshift. Recent theoretical work predicts that these horizon properties imprint a short-lived "direct wave" in the gravitational signal, emitted around the peak of a binary black hole merger. We present the first observational evidence for such a direct wave in GW250114, the loudest binary black hole merger detected to date. The signal oscillates at nearly twice the horizon rotation frequency and decays at a rate set by the surface gravity, matching theoretical predictions. This discovery opens a new observational window onto the near-horizon region, enabling direct measurement of black hole horizon properties from gravitational waves.