Analysis of Crab X-ray Polarization using Deeper IXPE Observations

TL;DR

This study provides detailed X-ray polarization measurements of the Crab pulsar and nebula using extended IXPE observations, revealing complex polarization patterns and magnetic field structures that suggest different emission mechanisms and jet dynamics.

Contribution

It presents the most comprehensive IXPE polarization analysis of the Crab to date, uncovering new phase-resolved and spatial polarization features with implications for emission models.

Findings

Detection of polarization in more pulsar phase bins.

Identification of an S-shaped polarization angle sweep.

Revelation of a toroidal magnetic field in the nebula.

Abstract

We present Crab X-ray polarization measurements using IXPE data with a total exposure of 300ks, three times more than the initial 2022 discovery paper. Polarization is detected in three times more pulsar phase bins, revealing an S-shaped $+40^\circ$ polarization angle sweep in the main pulse and ${>}1\sigma$ departures from the OPTIMA optical polarization in both pulses, suggesting different radiation mechanisms or sites for the polarized emission at the two wavebands. Our polarization map of the inner nebula reveals a toroidal magnetic field, as seen in prior IXPE analyses. Along the southern jet, the magnetic field orientation relative to the jet axis changes from perpendicular to parallel and the polarization degree decreases by ${\sim}6\%$. These observations may be explained by kink instabilities along the jet or a collision with a dense, jet-deflecting medium at the tip. Using spectropolarimetric analysis, we find asymmetric polarization in the four quadrants of the inner nebula, as expected for a toroidal field geometry, and a spatial correlation between polarization degree and photon index.