Plasma-photon interaction in curved spacetime I: formalism and quasibound states around nonspinning black holes

TL;DR

This paper develops a formalism for electromagnetic fields in plasma around black holes, revealing plasma-driven quasibound states that may lead to superradiant instabilities, advancing understanding of plasma-photon interactions in astrophysics.

Contribution

It introduces a comprehensive formalism for plasma-photon interactions in curved spacetime, highlighting new quasibound states and their potential instabilities near black holes.

Findings

Discovery of plasma-driven quasibound electromagnetic states.

Identification of conditions for superradiant instability.

Comparison showing differences from Proca and scalar field cases.

Abstract

We investigate the linear dynamics of an electromagnetic field propagating in curved spacetime in the presence of plasma. The dynamical equations are generically more involved and richer than the effective Proca equation adopted as a model in previous work. We discuss the general equations and focus on the case of a cold plasma in the background of a spherically-symmetric black hole, showing that the system admits plasma-driven, quasibound electromagnetic states that are prone to become superradiantly unstable when the black hole rotates. The quasibound states are different from those of the Proca equation and have some similarities with the case of a massive scalar field, suggesting that the linear instability can be strongly suppressed compared to previous estimates. Our framework provides the first step towards a full understanding of the plasma-photon interactions around astrophysical black holes.