Superradiance: Axionic Couplings and Plasma Effects

TL;DR

This paper investigates how axionic couplings influence superradiance around black holes, revealing a new stationary electromagnetic flux state and plasma-induced instabilities driven by axionic interactions.

Contribution

It introduces a numerical relativity study of the coupled axion-Maxwell system around black holes, uncovering a novel stationary electromagnetic flux state and plasma instabilities controlled by axionic couplings.

Findings

Discovery of a stationary electromagnetic flux state fed by superradiance.

Electromagnetic instabilities depend on axionic coupling strength.

High couplings trigger instabilities even in dense plasmas.

Abstract

Spinning black holes can transfer a significant fraction of their energy to ultralight bosonic fields via superradiance, condensing them in a co-rotating structure or "cloud". This mechanism turns black holes into powerful particle detectors for bosons with extremely feeble interactions. To explore its full potential, the couplings between such particles and the Maxwell field in the presence of plasma need to be understood. In this work, we study these couplings using numerical relativity. We first focus on the coupled axion-Maxwell system evolving on a black hole background. By taking into account the axionic coupling concurrently with the growth of the cloud, we observe for the first time that a new stage emerges: that of a stationary state where a constant flux of electromagnetic waves is fed by superradiance, for which we find accurate analytical estimates. Moreover, we show that the existence of electromagnetic instabilities in the presence of plasma is entirely controlled by the axionic coupling; even for dense plasmas, an instability is triggered for high enough couplings.