Shadow of black hole surrounded by magnetized plasma: Axion-plasmon cloud

TL;DR

This study investigates how an axion-plasmon cloud around a rotating black hole influences its shadow's shape and size, revealing potential observational signatures of axions in black hole environments.

Contribution

The paper introduces a model analyzing the impact of axion-plasmon clouds on black hole shadows, highlighting distinguishable effects from non-magnetized plasma and vacuum.

Findings

Axion-plasmon clouds alter the shadow shape and size of rotating black holes.

Heavier axions subtly affect the black hole shadow.

Energy emission rates increase with axion-plasmon coupling in rotating black holes.

Abstract

By exploiting the extreme environment of the black hole (BH) as a potential place for axion-photon interaction, we use an axion-producing model of the magnetized plasma to study the shadow of an asymptotically flat rotating BH immersed into an axion-plasmon cloud. By aiming to reveal footprints of axion in the dark shadow of BH, we in this paper explore the influence of the fixed axion-plasmon background on the motion of incident photons around the rotating BH. Under some free parameter settings, we find that axion-plasmon cloud around rotating BH affects the shape and size of the shadow in such a way that its role is distinguishable from non-magnetized plasma and standard vacuum solutions. By being limited to high rotation BH, we show that the size of the BH shadow increases as the axion-plasmon coupling gets strong. Interestingly, our analysis indicates that as the mass of axion gets heavier, it can leave a subtle imprint of itself on the shadow. Conversely, in the non-rotating limit (Schwarzschild), by recovering the spherical symmetry of the shadow shape of BH, its size decreases. In coordination with the trend of change in shadow size, the investigation of the energy emission from the BH surrounded by the magnetized plasma shows that the maximal energy emission rate from the rotating BH in the presence of axion-plasmon cloud increases compared to the non-magnetized plasma and the vacuum solutions. Subsequently, by relaxing the rotation, the axion-plasmon cloud causes a decrease in the energy emission rate from the BH.