Dipole Radiation and Beyond from Axion Stars in Electromagnetic Fields

TL;DR

This paper studies photon emission from axion stars in electromagnetic fields, revealing different behaviors depending on the coupling strength, including analytic formulas, non-monotonic luminosity, and exponential growth due to parametric resonance, supported by lattice simulations.

Contribution

It provides a comprehensive analysis of axion star photon emission across coupling regimes, including new analytic formulas and non-perturbative simulation results.

Findings

Analytic formula for dipole radiation at small coupling

Non-monotonic luminosity behavior with coupling strength

Exponential luminosity growth due to parametric resonance at large coupling

Abstract

We investigate the production of photons from coherently oscillating, spatially localized clumps of axionic fields (oscillons and axion stars) in the presence of external electromagnetic fields. We delineate different qualitative behaviour of the photon luminosity in terms of an effective dimensionless coupling parameter constructed out of the axion-photon coupling, and field amplitude, oscillation frequency and radius of the axion star. For small values of this dimensionless coupling, we provide a general analytic formula for the dipole radiation field and the photon luminosity per solid angle, including a strong dependence on the radius of the configuration. For moderate to large coupling, we report on a non-monotonic behavior of the luminosity with the coupling strength in the presence of external magnetic fields. After an initial rise in luminosity with the coupling strength, we see a suppression (by an order of magnitude or more compared to the dipole radiation approximation) at moderately large coupling. At sufficiently large coupling, we find a transition to a regime of exponential growth of the luminosity due to parametric resonance. We carry out 3+1 dimensional lattice simulations of axion electrodynamics, at small and large coupling, including non-perturbative effects of parametric resonance as well as backreaction effects when necessary. We also discuss medium (plasma) effects that lead to resonant axion to photon conversion, relevance of the coherence of the soliton, and implications of our results in astrophysical and cosmological settings.