Introduction

Sensitive X-ray polarimetry promises to reveal unique and crucial information about physical processes in and structures of neutron stars, black holes, and ultimately all classes of X-ray sources. We do not review the astrophysical problems for which X-ray polarization measurements will provide new insights, as these will be discussed in some detail in many of the presentations at this conference.

Despite major progress in X-ray imaging, spectroscopy, and timing, there have been only modest attempts at X-ray polarimetry. The last such dedicated experiment, conducted by Bob Novick (Columbia University) over three decades ago, had such limited observing time (and sensitivity) that even ∼10% degree of polarization would not have been detected from some of the brightest X-ray sources in the sky. Statistically significant X-ray polarization was detected in only one X-ray source, the Crab Nebula.

History

The first positive detection of X-ray polarization was performed in a sounding-rocket experiment that viewed the Crab Nebula in 1971. Using the X-ray polarimeter on the Orbiting Solar Observatory (OSO)-8, this result was confirmed with a 19-σ detection (P = 19.2%±1.0%), conclusively proving the synchrotron origin of the X-ray emission.