GW250114 reveals signatures of post-merger black-hole horizon

The horizon of a black hole, the 'surface of no return', is characterized by its rotation frequency Ω_H and surface gravity κ. A striking signature is that any infalling object appears to orbit at Ω_H owing to frame dragging, while its emitted signals decay exponentially at a rate set by κ as a consequence of gravitational redshift. Recent theoretical work¹ predicts that gravitational waves from binary black-hole mergers carry direct imprints of the properties of the merger remnant in the form of a 'direct wave'. This gravitational-wave component oscillates near 2Ω_H, reflecting the horizon's frame dragging, and decays at an increasing rate characterized by κ, with additional screening from the black hole's spacetime. Here we report observational evidence of a direct wave in GW250114², with a 90% credible matched-filter signal-to-noise ratio of ${15.8}_{-0.5}^{+0.1}$ ( ${17.1}_{-0.4}^{+0.1}$ ) in the LIGO Hanford (Livingston) detector. The measured properties are in full agreement with theoretical predictions for a Kerr black hole. These findings establish an observational channel to directly measure frame-dragging effects in black-hole ergospheres and explore (near-)horizon physics in dynamical, strong-gravity regimes.