Detection of Axion Stars in Galactic Magnetic Fields

TL;DR

This work analyzes the linear stability of axion-star configurations formed from axion-like particles (ALPs) embedded in a magnetized intergalactic medium by deriving a coupled axion–photon dispersion relation that includes plasma effects through the plasma frequency $\omega_p$, collisional damping $\nu_c$, and the axion–photon coupling $\beta=g_{a\gamma\gamma}B_0$. By studying both the magnetohydrodynamic (MHD) limit ($|\omega|\ll\nu_c$) and the collisionless limit ($|\omega|\gg\nu_c$), and performing a second-order perturbative expansion in $\beta$ and $\nu_c$, the paper shows that ALP decay is extremely slow in the MHD regime, yielding an attenuation rate $\omega_I\approx -\beta^2/(2\sigma)$ and a lifetime $\tau\approx 2\sigma/\beta^2$, far exceeding the age of the Universe. In the non-MHD regime, significant ALP→photon conversion would require a near-resonance condition $\beta^2/(m_a^2-\omega_p^2) \sim 1$, which is highly fine-tuned in typical intergalactic plasmas and thus unlikely; numerical results support negligible conversion under realistic conditions. The study concludes that axion stars remain stable in ordinary intergalactic environments, though extreme magnetic fields (e.g., near magnetars) could modify this outcome.

Abstract

We perform a linear mode analysis of a uniformly distributed cloud of axion-like particles (ALPs) embedded in a magnetized intergalactic medium, in order to investigate the stability of axion stars under realistic astrophysical conditions. We find that when the frequency $ω$ of transverse waves is much smaller than the collision frequency $ν_c$ of the intergalactic plasma, the conversion of ALPs into photons occurs on timescales far longer than the age of the Universe, ensuring stability of the star. In the opposite regime, $ω\gg ν_c$, significant axion-to-photon conversion may occur if the condition $\tfrac{β^2}{m_a^2-ω_p^2} < 1$ is satisfied, where $β$ depends on the ALP--photon coupling and the magnetic field, $m_a$ is the ALP mass, and $ω_p$ is the plasma frequency. We have calculated up to second order in perturbations to compute the effect of an ALP star. Since the calculated value of parameter $β^2$ is extremely small in comparison with $ω^2_p$, we argue that the direct detection of an axion star is highly unlikely in experiments like NCLE. However, since the calculated $β$ is extremely small compared to $ω_p$, this requires an unrealistically fine-tuned coincidence between $m_a$ and $ω_p$. As a consequence we argue that that detection of Our results therefore suggest that axion stars remain stable in typical intergalactic environments, though extreme magnetic fields (e.g.\ near magnetars) may lead to different outcomes.