Photon enhancement in a homogeneous axion dark matter background
We study the propagation of photons in a homogeneous axion dark matter background. When the axion decay into two photons is stimulated, the photon field exhibits a parametric instability in a small bandwidth centered on one half of the axion mass. We estimate analytically the enhancement for both coherent and non-coherent axion fields and we find that this effect could be relevant in the context of miniclusters and galactic halos.
Axions have not been experimentally observed yet, so their existence is still in suspense, but because of their cosmological role as dark matter candidates, the experimental efforts to search for them in the corresponding parameter space have been intense. In the laboratory, axions are searched for using the coupling to two photons described by Eq. (2). Especially, static magnetic fields have been strongly implemented [13, 14, 15]. Some running experiments, such as haloscopes searches , already have access to the parameter space where they are cold dark matter candidates, and several new proposals plan to do so in the near future (ALPS-II , IAXO , HAYSTAC [19, 20], MADMAX [21, 22], among others).
In this paper we are interested in studying the effects on an incident electromagnetic plane wave traveling within a homogeneous axion dark matter background. We find an small window where the photon field experiences parametric resonance, we estimate the rate of the enhancement in coherent and non-coherent axion background and finally, we apply our results to astrophysical structures. Related topics were discussed recently in References [23, 24, 25], but they do not consider non-coherent axion fields. Studies about the propagation of photons in an axion dark matter background had already been performed a few years ago in order to account for their effects in optical experiments [26, 27], however the instability was not considered.
2 Electromagnetic plane wave in a homogeneous axion background
3 Enhancement in a coherent and non-coherent axion background
4 Possible astrophysical relevance
4.1 Axion miniclusters
4.2 Galactic halos
As a final remark, It is very important to make clear that in this manuscript we are only considering homogeneous axion backgrounds. Inhomogeneities could change partially or drastically our results.
We would like to thank Pierre Sikivie for important discussions and Paola Arias for comments. This work was supported by the Chilean Commission on Research, Science and Technology (CONICYT) under Grant 78180100 (Becas Chile, Postdoctorado).
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