Impact of a plasma on the relaxation of black holes

TL;DR

This paper explores how interstellar plasma affects black hole signals, revealing that plasma can obscure electromagnetic modes and generate detectable gravitational wave echoes near charged black holes.

Contribution

It demonstrates the significant impact of plasma on black hole electromagnetic signals and introduces the possibility of observing gravitational wave echoes caused by plasma near charged black holes.

Findings

Electromagnetic modes are completely screened by plasma near the black hole's light ring.

Plasma can induce long-lasting gravitational wave echoes in the ringdown phase.

Plasma effects could alter black hole spectroscopy and detection strategies.

Abstract

Our universe is permeated with interstellar plasma, which prevents propagation of low-frequency electromagnetic waves. Here, we show that two dramatic consequences arise out of such suppression; (i) if plasma permeates the light ring of a black hole, electromagnetic modes are screened entirely from the gravitational-wave signal, changing the black hole spectroscopy paradigm; (ii) if a near vacuum cavity is formed close to a charged black hole, as expected for near equal-mass mergers, ringdown "echoes" are excited. The amplitude of such echoes decays slowly and could thus serve as a silver bullet for plasmas near charged black holes.