2 - Scattering polarimetry in high-energy astronomy

Summary

At energies above a few keV, photon scattering provides an important means of measuring photon polarization. Here we review the fundamental principles of scattering polarimetry, present a summary of some of the more recent results, and review the prospects for new experimental results within the next few years.

Introduction

It has now been a little more than 100 years since the first reported laboratory measurements of γ-ray polarization based on the use of Compton scattering. Although the first efforts to apply this technique in high-energy (X-ray and γ-ray) astronomy took place almost 40 years ago, this area of research is still in its infancy. This is a notoriously difficult area of research, compounded by the combination of low flux levels, high background rates and instrumental artifacts that can often mimic a polarization signature. Nonetheless, all of the recent polarization measurements have relied on this approach.

Experimental considerations

Scattering polarimetry relies on experimental methods that are based on the scattering of photons off electrons. The scattering of photons off single electrons is variably referred to as Compton scattering or, at lower energies, as Thomson scattering. Thomson scattering is the classical limit of Compton scattering in which there is no loss of energy to the electron. At lower energies, coherent scattering off the atomic electron cloud can also be important.