33 - Gamma-ray polarimetry with SPI

Summary

Young energetic pulsars are capable of accelerating electrons to extremely high energies, which subsequently become visible in the X-ray and low-energy γ-ray domain through synchrotron radiation. The recent polarization measurement of the Crab γ-ray emission provides a powerful investigative tool for the physical conditions and geometry of the magnetic field close to the pulsar. The Crab nebula has been found to emit linearly polarized γ-rays during the off-pulse phase with an efficiency close to the maximum allowable by physics. The close alignment between the electric vector and the spin axis of the neutron star places severe boundaries on theoretical models. The off-pulse γ-radiation is constrained to originate somewhere close to the pulsar, but outside the light cylinder. Three contenders are identified: the striped-wind model; radiation from within the inner jet; Doppler-boosted radiation from knot-like features close to the pulsar.

Introduction

Observations in the γ-ray regime allow us to scrutinize some of the most energetic emission processes associated with cosmic sources. Unlike other wave bands that mainly show emission due to thermal reprocessing within hot gases, the majority of γ-ray emission is decidedly non-thermal in nature and is generally produced directly by electrons and other elementary particles in magnetic fields. The electrons which produce the γ-rays are extremely energetic and often at the upper limit of the capability of the accelerator that produces them.