New constraints on axion-like particles from IXPE polarization data for magnetars

We derive new constraints on axion-like particles (ALPs) using precision X-ray polarization studies of magnetars. Specifically, we use the first detection of polarized X-rays from the magnetars 4U 0142+61 and 1RXS J170849.0-400910 by the Imaging X-ray Polarimetry Explorer (IXPE) to place bounds on the product of the ALP-photon and ALP-nucleon couplings, g_aγg_aN, with g_aN being responsible for ALP production in the core of the magnetar and g_aγ controlling the ALP-photon conversion probability in the magnetosphere. These bounds are most sensitive to the magnetar core temperature, and we use two benchmark values of 1×10⁸ K and 5×10⁸ K to derive our constraints. For the latter choice, our bounds are competitive with the existing bounds on the coupling product coming from a combination of CAST (for g_aγ) and SN1987A (for g_aN). We advocate for more precise and extensive observational campaigns in the higher end of the 2–8 keV spectral window, where ALP-induced polarization is the strongest. We further advocate for hard X-ray polarization studies of young, hot, near-Earth magnetars with strong magnetic fields.