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.
Highlights
Axions and axion-like particles provide excellent dark matter candidates [1,2,3,4,5,6,7,8,9], as well as candidates for driving inflation [10,11,12,13] and, perhaps, even present day acceleration [14,15,16,17]
We investigate the production of photons from coherently oscillating, spatially localized clumps of axionic fields in the presence of external electromagnetic fields
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
Summary
Axions and axion-like particles provide excellent dark matter candidates [1,2,3,4,5,6,7,8,9], as well as candidates for driving inflation [10,11,12,13] and, perhaps, even present day acceleration [14,15,16,17]. A variety of experimental efforts are underway to detect axions and axion-like particles (ALPs) in the laboratory [25,26,27,28] and through their unique astrophysical and cosmological signatures [29,30,31,32,33,34,35,36,37,38] Many of these searches rely upon a coupling of the axion field φ(x, t) to electromagnetism via the interaction gaγφE · B. If φ is oscillating it can have an enhanced effect through resonances [39,40,41] With these considerations, it is natural to explore the impact of axion stars in strong electromagnetic fields. We do note that it might be sensitive to the fiducial parameters chosen
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