A recent change in the optical and γ-ray polarization of the Crab nebula and pulsar

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.