Horizon-redshift transfer in black-hole direct-wave damping

Abstract

Direct waves from black-hole mergers may probe horizon dynamics, but their observed envelopes need not decay at the Kerr surface-gravity rate. We compute the complex-frequency spin-$-2$, $\ell =m=2$ Teukolsky response and combine it with a finite-duration near-horizon source whose outgoing amplitude is suppressed by gravitational redshift. The screened Kerr response to this finite-duration source produces an observable envelope damping $\geff<\kap$. For GW250114, this corresponds to $\geff\simeq0.4~{\rm ms}^{-1}$, consistent with a joint H1--L1 analysis of QNM-subtracted residuals. As a consistency check, the GW231226 remnant parameters give $\geff\simeq0.31~{\rm ms}^{-1}$, compatible with the event's residual profile. These results identify direct-wave envelope damping as an observable of horizon-redshift transfer rather than a direct measurement of surface gravity.

13 pages, 5 figures