Introduction

A photon carries information about its direction, time of arrival, energy and polarization. Satellite missions such as Uhuru, Ariel V, HEAO-1 and ROSAT located hundreds of sources in their all-sky surveys, and missions such as HEAO-1, ASCA, Suzaku, RXTE, Chandra, and XMM/Newton characterized source spectra and time variability. The X-ray community now needs polarization information to reveal the intrinsic small-scale geometry of astrophysical systems and to evaluate physical processes in regions near compact objects which cannot be resolved via imaging, spectroscopy or timing.

X-ray radiation is polarized when the production mechanism has an implicit directionality, such as when electrons interact with a magnetic field to produce cyclotron and synchrotron emission. In radio quasars, for example, the amplitude of the polarization is a diagnostic for the X-ray emission mechanism and the polarization direction associates the X-ray emission with particular regions identified in milli-arc-second radio images. Polarization also occurs when X-rays are scattered by electrons, a common process in the highly ionized environments of compact X-ray sources.