Detection of Axion Stars in Galactic Magnetic Fields

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 $\omega$ of transverse waves is much smaller than the collision frequency $\nu_c$ 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, $\omega \gg \nu_c$, significant axion-to-photon conversion may occur if the condition $\tfrac{\beta^2}{m_a^2-\omega_p^2} < 1$ is satisfied, where $\beta$ depends on the ALP--photon coupling and the magnetic field, $m_a$ is the ALP mass, and $\omega_p$ 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 $\beta ^2$ is extremely small in comparison with $\omega^2_p$, we argue that the direct detection of an axion star is highly unlikely in experiments like NCLE. However, since the calculated $\beta$ is extremely small compared to $\omega_p$, this requires an unrealistically fine-tuned coincidence between $m_a$ and $\omega_p$. 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.