2026/07/18 by Soumya Samrat Mandal, Maxim Lyutikov
#astro-ph.HE #gr-qc
We describe a classical (non-quantum) radiation process: additional (to Hawking) emission by a black hole evaporating in an external magnetic field in vacuum. The electromagnetic radiation process is completely electric charge-free and bears some resemblance to the Gertsenshtein-Zel'dovich effect. The time evolution of the spacetime metric perturbs a static background magnetic field, inducing a radiative field that acts as an effective electromagnetic source even in the absence of physical charges or currents. To isolate the dynamic effects of the time-dependent spacetime on the external magnetic field, we approximate Hawking radiation as a spherically symmetric outflow of null fluid governed by the prescribed time-dependence of the central mass M(t). We employ Laplace transform, which selects the retarded outgoing branch of the electromagnetic response, producing fields proportional to Θ(t-r) and thereby fixing a causal radiative arrow of emission. The emitted spectral energy is red-dominated, scaling as dEB/dω∝ B02 m02 τH-2/3 ω-8/3, where τH is the Hawking evaporation timescale, and corresponds to a pure Transverse Electric (TE) mode. There is no final bright burst at the end of the evaporation. We offer classical analogues for this mechanism as transmission-line emission and, separately, as the displacement current emission from a medium with time-varying dielectric permittivity.