Superradiance in massive vector fields with spatially varying mass

TL;DR

This study investigates superradiance of massive vector fields, like photons in plasma, around black holes, showing that spatial mass variations and plasma density profiles significantly influence the growth of superradiant clouds.

Contribution

It extends previous scalar superradiance studies to the spin-1 case using relativistic simulations with spatially varying mass profiles.

Findings

Superradiance is suppressed by spatially varying effective mass.

Thick plasma disks are more conducive to superradiant growth.

Leaking of the superradiant cloud occurs in thin disk profiles.

Abstract

Superradiance is a process by which massive bosonic particles can extract energy from spinning black holes, leading to the build up of a "cloud" if the particle has a Compton wavelength comparable to the black hole's Schwarzschild radius. One interesting possibility is that superradiance may occur for photons in a diffuse plasma, where they gain a small effective mass. Studies of the spin-0 case have indicated that such a build up is suppressed by a spatially varying effective mass, supposed to mimic the photons' interaction with a physically realistic plasma density profile. We carry out relativistic simulations of a massive Proca field evolving on a Kerr background, with modifications to account for the spatially varying effective mass. This allows us to treat the spin-1 case directly relevant to photons, and to study the effect of thinner disk profiles in the plasma. We find similar qualitative results to the scalar case, and so support the conclusions of that work: either a constant asymptotic mass or a shell-like plasma structure is required for superradiant growth to occur. We study thin disks and find a leakage of the superradiant cloud that suppresses its growth, concluding that thick disks are more likely to support the instability.