A recent change in the optical and {\gamma}-ray polarization of the Crab nebula and pulsar

TL;DR

This study reports simultaneous optical and gamma-ray polarization changes in the Crab nebula and pulsar, suggesting magnetic reconnection processes, and introduces the first measurement of optical circular polarization from this system.

Contribution

It presents the first combined optical and gamma-ray polarization observations of the Crab system, revealing correlated polarization angle changes and evidence for magnetic reconnection.

Findings

Optical polarization angle changed from 109.5° to 85.3° between 2005 and 2012.

Gamma-ray polarization angle changed from 115° to 80° between 2003-2007 and 2012-2014.

First measurement of non-zero optical circular polarization from the Crab pulsar+knot system.

Abstract

We report on observations of the polarization of optical and {\gamma}-ray photons from the Crab nebula and pulsar system using the Galway Astronomical Stokes Polarimeter (GASP), the Hubble Space Telescope/Advanced Camera for Surveys (HST/ACS) and the International Gamma-Ray Astrophysics Laboratory satellite (Integral). These, when combined with other optical polarization observations, suggest that the polarized optical emission and {\gamma}-ray polarization changes in a similar manner. A change in the optical polarization angle has been observed by this work, from 109.5 \pm 0.7\deg in 2005 to 85.3 \pm 1.4 \deg in 2012. On the other hand, the {\gamma}-ray polarization angle changed from 115 \pm 11 \deg in 2003-2007 to 80 \pm 12 \deg in 2012-2014. Strong flaring activities have been detected in the Crab nebula over the past few years by the high energy {\gamma}-ray missions Agile and Fermi, and magnetic reconnection processes have been suggested to explain these observations. The change in the polarized optical and {\gamma}-ray emission of the Crab nebula/pulsar as observed, for the first time, by GASP and Integral may indicate that reconnection is possibly at work in the Crab nebula. We also report, for the first time, a non-zero measure of the optical circular polarization from the Crab pulsar+knot system.