GW250114 reveals black hole horizon signatures

Abstract: The horizon of a black hole, the "surface of no return," is characterized by its rotation frequency $\Omega_H$ and surface gravity $\kappa$. A striking signature is that any infalling object appears to orbit at $\Omega_H$ due to frame dragging, while its emitted signals decay exponentially at a rate set by $\kappa$ as a consequence of gravitational redshift. Recent theoretical work predicts that the merger phase of gravitational waves from binary black hole coalescences carries direct imprints of the remnant horizon's properties, via a "direct wave" component that (i) oscillates near $2\Omega_H$, reflecting the horizon's frame dragging and the quadrupole nature of the gravitational radiation, and (ii) decays at an increasing rate characterized by $\kappa$, with additional screening from the black hole's potential barrier. In this paper, we report observational evidence for the direct wave in GW250114 with a matched-filter signal-to-noise ratio of $14.0^{+0.2}_{-0.1}$ ($13.5^{+0.1}_{-0.2}$) in the LIGO Hanford (Livingston) detector. The measured properties are in full agreement with theoretical predictions. These findings establish a new observational channel to directly measure frame-dragging effects in black hole ergospheres and explore (near-)horizon physics in dynamical, strong-gravity regimes.