Polarized radio emission from a magnetar

We present polarization observations of the radio emitting magnetar AXP J1810-197. Using simultaneous multifrequency observations performed at 1.4, 4.9 and 8.4 GHz, we obtained polarization information for single pulses and the average pulse profile at several epochs. We find that in several respects this magnetar source shows similarities to the emission properties of normal radio pulsars while simultaneously showing striking differences. The emission is nearly 80-95 per cent polarized, often with a low but significant degree of circular polarization at all frequencies which can be much greater in selected single pulses. The position angle swing has a low average slope of only 1 deg deg(-1), deviating significantly from an S-like swing as often seen in radio pulsars which is usually interpreted in terms of a rotating vector model and a dipolar magnetic field. The observed position angle is consistent at all frequencies while showing significant secular variations. On average, the interpulse is less linearly polarized but shows a higher degree of circular polarization. Some epochs reveal the existence of non-orthogonal emission modes in the main pulse and systematic wiggles in the PA swing, while the interpulse shows a large variety of position angle values. We interpret many of the emission properties as propagation effects in a non-dipolar magnetic field configuration where emission from different multipole components is observed.