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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.