General History of X-ray Polarimetry in Astrophysics

Soon after the discovery of the first extrasolar X-Ray sources, it was suggested that polarimetry could play a major role as a diagnostic tool. Attempts to measure polarization of X-ray sources were performed by the team of Columbia University lead by Robert Novick. The technique of Bragg diffraction at 45∘ was successful to detect the polarization of the Crab with rockets and with OSO-8 satellite. In the following evolution of X-ray Astronomy, polarimetry was too mismatched with the improved sensitivity of imaging and spectroscopy, based on the use of optics. As a consequence, no polarimeter was flown any more. At the beginning of the century, a new class of instruments based on the photoelectric effect was developed. In the focus of an X-ray telescope, they can perform angular, energy, and time resolved polarimetry and benefit of the large increase of sensitivity due to the optics. The Imaging X-Ray Polarimetry Explorer, exploiting this technique, was launched at the end of 2021.