Mechanics of non-Killing horizons

TL;DR

This paper explores the mechanics and thermodynamics of non-Killing horizons in rotating spacetimes beyond general relativity, defining new surface gravity notions and analyzing their implications for black hole laws.

Contribution

It introduces and compares three surface gravity definitions for non-Killing horizons, derives a modified Smarr formula, and examines Hawking radiation in these contexts.

Findings

Inaffinity and normal surface gravities generally differ.

Normal and peeling surface gravities always coincide.

Hawking temperature is governed by the peeling surface gravity.

Abstract

We investigate the mechanics of stationary axisymmetric non-Killing horizons, which emerge in spacetimes that do not enjoy the symmetry known as circularity -- as is commonly the case for rotating black holes beyond general relativity. Specifically, we define and compute three notions of surface gravity: inaffinity, normal, and peeling; and find that the inaffinity and normal definitions generically differ, while the normal and peeling definitions always agree, although none of them is constant over the horizon. We then derive a version of Smarr's formula, which appears to involve an average over the horizon of the normal surface gravity. We also compute, via the tunnelling method, the spectrum of Hawking's radiation, verifying that its temperature is controlled by the (non-constant) peeling surface gravity. Finally, we recapitulate the status of the four laws of black hole mechanics in situations in which the event horizon fails to be Killing. Our results thus pave the way to a deeper understanding of black hole thermodynamics beyond general relativity.