Optical Polarimetry of the Inner Crab Nebula and Pulsar

TL;DR

This study presents high-resolution optical polarisation measurements of the Crab Nebula and pulsar, providing new insights into their emission regions and confirming the synchrotron knot as the main source of the pulsar's DC component.

Contribution

First high-spatial resolution, multi-epoch polarisation study of the inner Crab Nebula and pulsar, measuring polarisation vectors and confirming the knot as the main DC component source.

Findings

Pulsar's degree of linear polarisation: 5.2%

Pulsar's position angle: 105.1°

Synchrotron knot's polarisation: 59.0%

Abstract

Time-resolved polarisation measurements of pulsars offer an unique insight into the geometry of their emission regions. Such measure\ ments provide observational constraints on the different models proposed for the pulsar emission mechanisms. Optical polarisation data of the Crab Nebula was obtained from the HST archive. The data set consists of a series of observations of the nebula taken with the HST/ACS. We produced polarisation vector maps of the inner nebula and measured, for the first time, the degree of linear polarisation (P.D.) and the position angle (P.A.) of the pulsar's integrated pulse beam, and of its nearby synchrotron knot. This yielded $\rm P.D.=5.2\pm0.3%$ and $\rm P.A.=105.1\pm1.6\degr$ for the pulsar, and $\rm P.D.=59.0\pm1.9%$ and $\rm P.A.=124.7\pm1.0\degr$ for the synchrotron knot. This is the first high-spatial resolution multi-epoch study of the polarisation of the inner nebula and pulsar. None of the main features in the nebula show evidence of significant polarisation evolution in the period covered by these observations. The results for the pulsar are consistent with those obtained by S{\l}owikowska et al (2009) using the high-time resolution photo-polarimeter OPTIMA, once the DC component has been subtracted. Our results clearly prove that the knot is the main source of the DC component.