Measuring a Black Hole's Area Immediately after Merger: A Direct-Wave Test of Hawking's Area Law

Abstract

Black-hole area is the geometric variable behind horizon thermodynamics. We introduce a gravitational-wave method to infer a Kerr-equivalent horizon area from direct waves in the near-merger signal, before quasinormal ringing dominates at late times. Applied to GW250114, and interpreting the fitted direct-wave frequency and damping rate as horizon quantities, we find that analyses initiated $3$--$4.5M$ before the peak-amplitude time yield an area consistent with the Kerr remnant. This result gives a first area measurement using direct waves and a new near-merger test of Hawking's area law.