2022ApJ...930....1L

We present a polarimetric study of the pulsar wind nebula (PWN) in supernova remnant G21.5-0.9, using archival Very Large Array data taken at 5 and 7.4 GHz. The rotation measure (RM) map of the PWN shows a symmetric pattern that aligns with the presumed pulsar spin axis direction, implying a significant contribution to the RM from the nebula. We suggest that the spatial variation of the internal RM is mostly caused by the nonuniform distribution of electrons originating from the supernova ejecta. Our high-resolution radio polarization map reveals a global radial B-field. We show that a simple model with overall radial field and turbulence on a small scale can reproduce many observed features of the PWN, including the polarization pattern and polarized fraction. The modeling results also reject a strong large-scale toroidal B-field, suggesting that the toroidal field observed in the inner PWN cannot propagate to the entire nebula. Lastly, our model predicts that the internal Faraday rotation would break the linear relation between the polarization angle and the square of the wavelength, and cause severe depolarization at low frequencies.