Radio polarization study of the young pulsar wind nebula in supernova remnant G21.5-0.9

Abstract

Pulsars carry strong magnetic fields and rotate very rapidly. They are usually modelled as a rotating magnetic dipole which generates strong electric potential and accelerates charged particles to relativistic speed. Shock acceleration and magnetic reconnection also play an important role in accelerating the pulsar wind particles. The relativistic particles surrounding the pulsar forms the pulsar wind nebula (PWN), mainly emitting in synchrotron radiation. PWNe can be classified into different evolutionary stages and we focus on young PWNe in this work.

Young PWNe share many common features in their morphology, spectrum, etc. Their magnetic field structure however shows a large diversity. The magnetic field is believed to be toroidal near the pulsar at least before the termination shock, but the magnetic field develops into distinctly different configuration after the shock as shown in observations. It is important to understand the evolution of the magnetic field as it relates to our knowledge in shock physics, magnetohydrodynamics and magnetosphere of a pulsar. The PWN in G21.5-0.9 is a peculiar case that has a radial magnetic field, which makes it an interesting PWN to study.

G21.5-0.9 is a composite supernova remnant with a PWN at the center. It is unique for its young age and radial magnetic field. In this work, we present a polarimetric study of the PWN 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 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. By simulating the Faraday complexity and depolarization caused by internal Faraday rotation, We suggest that an L band observation is important in testing the model.

The infrared observation probed the inner part of G21.5-0.9 and found a toroidal magnetic field. The result is consistent with the theoretical paradigm and is of interest to know how the toroidal field turns into a radial field in large scale. This problem can hopefully be answered with future simulations and future X-ray polarimeters (e.g., IXPE).