Plasma-photon interaction in curved spacetime II: collisions, thermal corrections, and superradiant instabilities

TL;DR

This paper investigates how plasma interactions around black holes influence electromagnetic field behavior, revealing conditions for mode stability, superradiant instabilities, and resonant phenomena in curved spacetime.

Contribution

It extends previous models by including plasma collisions, thermal effects, and black hole spin, providing new insights into plasma-driven electromagnetic modes near black holes.

Findings

Long-lived plasma modes persist with realistic collision timescales.

Thermal effects do not impact axial long-lived modes.

Spinning black holes induce superradiant instabilities in plasma modes.

Abstract

Motivated by electromagnetic-field confinement due to plasma near accreting black holes, we continue our exploration of the linear dynamics of an electromagnetic field propagating in curved spacetime in the presence of plasma by including three effects that were neglected in our previous analysis: collisions in the plasma, thermal corrections, and the angular momentum of the background black-hole spacetime. We show that: (i) the plasma-driven long-lived modes survive in a collisional plasma except when the collision timescale is unrealistically small; (ii) thermal effects, which might be relevant for accretion disks around black holes, do not affect the axial long-lived modes; (iii) in the case of a spinning black hole the plasma-driven modes become superradiantly unstable at the linear level; (iv) the polar sector in the small-frequency regime admits a reflection point due to the resonant properties of the plasma. Dissipative effects such as absorption, formation of plasma waves, and nonlinear dynamics play a crucial role in the vicinity of this resonant point.