Not yet reviewed by Pith; the record is open.

Axion-like-particles (ALPs) produced in the core of a magnetar can convert to photons in the magnetosphere, giving rise to novel features in the X-ray spectrum. Since ALPs only mix with the parallel mode of the photon, the polarization of the soft and hard X-ray spectra is predicted to have an O-mode component, in addition to the mainly X-mode component given by most astrophysical models. The relative strength of the O-mode component depends on the intensity of ALPs produced in the core and the probability of conversion. We quantify our results by considering X-ray emission produced both by astrophysical processes and by ALP-photon conversion, in an uncorrelated fashion, and in different relative proportions, which we parametrize by the angle $\chi_0$. We then define a normalized astrophysics-subtracted Stokes parameter $R$ which only acquires non-zero values in the presence of ALP-photon conversion. We find, remarkably, that the parameter $R$ factorizes into a product of the ALP-to-photon conversion probability and $\cos(2\chi_0)$ and display $R$, as well as the usual Stokes parameter $Q$, as a function of the photon energy and relative fractions of ALP and photon intensities. For benchmark points currently allowed by the CAST experiment, the O-mode prediction can be tested in future X-ray polarimeters and used either to constrain ALPs or find evidence for them.