Electromagnetic Signatures of Dark Photon Superradiance

TL;DR

This paper explores how ambient electrons can suppress dark photon superradiance around black holes, affecting electromagnetic signals and providing new insights into constraints on ultra-light bosons and the Standard Model photon.

Contribution

It investigates the quenching of dark photon superradiance by scattering with electrons, revealing conditions under which superradiance is suppressed or allowed, and predicts observable electromagnetic signatures.

Findings

Superradiance can be quenched for dark photon masses above 10^{-17} eV with ambient electrons.

In-medium suppression of kinetic mixing affects superradiance efficiency depending on mass and electron density.

Electromagnetic signals from superradiance could reach luminosities up to 10^{57} erg/s, detectable by current telescopes.

Abstract

Black hole superradiance is a powerful tool in the search for ultra-light bosons. Constraints on the existence of such particles have been derived from the observation of highly spinning black holes, absence of continuous gravitational-wave signals, and of the associated stochastic background. However, these constraints are only strictly speaking valid in the limit where the boson's interactions can be neglected. In this work we investigate the extent to which the superradiant growth of an ultra-light dark photon can be quenched via scattering processes with ambient electrons. For dark photon masses $m_{\gamma^\prime} \gtrsim 10^{-17}\,{\rm eV}$, and for reasonable values of the ambient electron number density, we find superradiance can be quenched prior to extracting a significant fraction of the black-hole spin. For sufficiently large $m_{\gamma^\prime}$ and small electron number densities, the in-medium suppression of the kinetic mixing can be efficiently removed, and quenching occurs for mixings $\chi_0 \gtrsim \mathcal{O}(10^{-8})$; at low masses, however, in-medium effects strongly inhibit otherwise efficient scattering processes from dissipating energy. Intriguingly, this quenching leads to a time- and energy-oscillating electromagnetic signature, with luminosities potentially extending up to $\sim 10^{57}\,{\rm erg / s}$, suggesting that such events should be detectable with existing telescopes. As a byproduct we also show that superradiance cannot be used to constrain a small mass for the Standard Model photon.