Radio emission from the stimulated decay of axion-like particle condensates

In the past few years, the search for axion-like particles (ALPs) has grown significantly due to their potential to account for the total abundance of the cold dark matter (CDM) content in the universe. It has been recently pointed out that ALPs may form a Bose-Einstein condensate (BEC) and, through their gravitational attraction and self-interactions, they can thermalize to spatially localized clumps. Naturally, the coupling between ALPs and photons allows the spontaneous decay of ALPs into pairs of photons. For ALP condensates with very high occupation numbers, the stimulated decay of ALPs is also possible, and thus the photon occupation number can receive Bose enhancement and grow exponentially. The expansion of the universe and the plasma effects can disrupt this extremely fast process. In this work, we quantify the effect of the cosmic plasma in modifying the photon growth profile in the presence of an ALP background. Then we examine the consequences on the detectability of the radio emissions produced from this process by the forthcoming radio telescopes such as the Square Kilometer Array (SKA) and MeerKAT telescopes with the intention of detecting the CDM ALPs. We find that neither the current cosmic plasma nor the plasma in the galactic halos can prevent the stimulated decay of ALP with the mass range $10^{-11} \text{--} 10^{-4} \ \text{eV}$. Additionally, non-observation of the radio signal produced via the stimulated decay of ALPs in the mass range of $4.96 \times 10^{-7} \text{--} 1.04 \times 10^{-4} \ \text{eV}$ would result in upper limits on the ALP-photon $g_{a\gamma}$ in the range of $5.44 \times 10^{-12} \text{--} 1.03 \times 10^{-9} \; \text{GeV}^{-1}$ with the next-generation of the SKA and MeerKAT radio telescopes.