Polarimetry is widely considered a powerful observational technique in X-ray astronomy, useful to enhance our understanding of the emission mechanisms, geometry and magnetic field arrangement of many compact objects. However, the lack of suitable sensitive instrumentation in the X-ray energy band has been the limiting factor for its development in the last three decades. Up to now, polarization measurements have been made almost exclusively with Bragg crystal or Thomson scattering techniques and so far the only unambiguous detection of X-ray polarization has been obtained by the Weisskopf group in 1976 from observations of the Crab Nebula. Only recently, with the development of a new class of high-sensitivity imaging detectors, the possibility to exploit the photoemission process to measure the photon polarization has become a reality. This paper will review the history of X-ray photoelectric polarimetry and discuss some innovative experimental techniques.
X-ray astronomy deals with the most violent and compact spots in the Universe, such as the surfaces of pulsars, the close orbits around giant black holes, and the blast waves of supernova explosions. By making efficient use of the few photons emitted by disks around black holes and other objects, astronomers have successfully applied photometry, imaging and spectroscopy to these energetic and often variable sources. But polarimetry has been largely ignored at X-ray wavelengths because of the inefficiency of the existing instruments.