34 - INTEGRAL/IBIS observations of the Crab nebula and GRB 041219A polarization

Summary

Neutron stars generate powerful winds of relativistic particles that form bright synchrotron nebulae around them. Polarimetry provides a unique insight into the geometry and magnetic configuration of the wind, but high-energy measurements have failed until recently. The INTEGRAL-IBIS telescope has been used in its Compton mode to search for linearly polarized emission for energies above 200 keV from the Crab nebula. The asymmetries in the instrument response are small and we obtain evidence for a strongly polarized signal at an angle parallel to the pulsar rotation axis. This result confirms the detection recently reported by, and extends the polarization measure for all the pulsar's phases. We also report the recent observation of a variable polarization signal from the long GRB 041219A. The achieved sensitivity opens a new window for polarimetric studies at energies above 200 keV.

Introduction

Gamma-ray polarimetry has been possible with Compton telescopes since the 1970s. Photons that are Compton scattered between two detectors follow an azimuthal distribution around the source direction that allows quantification of the degree and direction of linear polarization because the photon is preferentially scattered in a plane at right angles to its incident electric vector. It was unsuccessful until recently because of intrinsic asymmetries in the detector response and nonuniformities in the large background signals. They induce pseudo-polarimetric signals, even from an unpolarized source, that limit the sensitivity to any detection.