Magnetic Field Structure and Faraday Rotation of the Plerionic Supernova Remnant G21.5$-$0.9

TL;DR

This study uses radio polarization data to analyze the magnetic field structure and internal Faraday rotation in the PWN G21.5$-$0.9, revealing a predominantly radial magnetic field and the influence of supernova ejecta on electron distribution.

Contribution

It provides the first detailed polarization map of G21.5$-$0.9 and introduces a simple model supporting a radial magnetic field with turbulence, challenging the idea of a large-scale toroidal field.

Findings

The RM map shows symmetry aligned with the pulsar spin axis.

The magnetic field is predominantly radial, not toroidal.

The model reproduces polarization features and suggests turbulence and a radial B-field.

Abstract

We present a polarimetric study of the pulsar wind nebula (PWN) in supernova remnant G21.5$-$0.9 using archival Very Large Array (VLA) data. 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 of RM from the nebula. We suggest that the spatial variation of the internal RM is mostly caused by non-uniform distribution of electrons originated from the supernova ejecta. Our high-resolution radio polarization map reveals an overall radial $B$-field. We construct a simple model with an overall radial $B$-field and turbulence in small scale. The model can reproduce many of the observed features of the PWN, including the polarization pattern and polarized fraction. The results also reject a large-scale toroidal $B$-field which implies that the toroidal field observed in the inner PWN cannot propagate to the entire nebula.