Axion-Like Particles, Magnetars, and X-ray Astronomy

If they exist, axion-like particles (ALPs) can be produced in the core of magnetars. As they propagate, they then convert to photons in the magnetosphere, leading to possible signatures in the X-ray photon spectrum. Moreover, since ALPs only mix with the parallel mode of the photon, interesting modifications to the photon polarization are also possible. We determine these effects by computing the Stokes parameters in the presence of ALPs and show that the relevant quantity factorizes into a product of the ALP-to-photon conversion probability and a factor that depends on the initial mixture of ALPs and photons at the surface of the magnetar.