Magnetic Field Structure of the Crab Pulsar Wind Nebula Revealed with IXPE

TL;DR

This study uses IXPE X-ray polarization data to map the magnetic field structure of the Crab pulsar wind nebula, revealing the magnetic field orientation, turbulence level, and polarization features near the pulsar and jet.

Contribution

It provides the first detailed polarization-based magnetic field map of the Crab nebula, including turbulence estimates and the identification of a low polarization region linked to optical filaments.

Findings

Magnetic field near the pulsar is toroidal and perpendicular to the spin axis.

A low polarization region coincides with an optical filament, suggesting wind deflection.

Turbulent magnetic fields constitute about two-thirds of the total magnetic energy.

Abstract

We report a detailed study of the magnetic-field structure of the Crab pulsar wind nebula, using the X-ray polarization data in 2--8~keV obtained with the Imaging X-ray Polarimetry Explorer. Contamination of the pulsar emission to the data of the nebula region was removed through application of a stringent pulsation phase-cut, extracting a phase range of 0.7--1.0 only. We found that the electric field vector polarization angle (PA) was about $130^{\circ}$ from north to east with the polarization degree (PD) of about 25\% at the pulsar position, indicating that the direction of the toroidal magnetic field is perpendicular to the pulsar spin axis in the region close to the termination shock. The PA gradually deviated from the angle as an increasing function of the distance from the pulsar. There was a region of a low PD to the west of the X-ray torus. Although such a region is expected to be located at the torus edge, where geometrical depolarization due to a steep spatial variation of the PA is expected, the observed low-PD region positionally deviated from the edge. We found that the region of low PD positionally coincided with a dense filament seen in the optical band, and conjecture that the low-PD region may be produced through deflection of the pulsar wind. By comparing the values of the PD at the pulsar position between the data and a model, in which toroidal and turbulent magnetic fields were considered, we estimated the fractional energy of the turbulent magnetic field to be about $2/3$ of the total. We also evaluated a potential polarization of the northern jet in the nebula and derived the PD and PA to be about $30\%$ and $120^{\circ}$, respectively.