Dipole Radiation and Beyond from Axion Stars in Electromagnetic Fields
  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.
