GW250114 reveals black hole horizon signatures

Abstract

The horizon of a black hole, the "surface of no return", is characterized by its rotation frequency $\hat{I}{\text{c◯}}_H$ and surface gravity $\hat{I}º$. A striking signature is that any infalling object appears to orbit at $\hat{I}{\text{c◯}}_H$ due to frame dragging, while its emitted signals decay exponentially at a rate set by $\hat{I}º$ 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 $2\hat{I}{\text{c◯}}_H$, reflecting the horizon's frame dragging and the dominant quadrupole nature of the gravitational radiation, and (ii) decays at an increasing rate characterized by $\hat{I}º$, 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 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. 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.