  The Gas Pixel Detector, recently developed and continuously improved by Pisa
INFN in collaboration with IASF-Roma of INAF, can visualize the tracks produced
within a low Z gas by photoelectrons of few keV. By reconstructing the impact
point and the original direction of the photoelectrons, the GPD can measure the
linear polarization of X-rays, while preserving the information on the
absorption point, the energy and the time of individual photons. Applied to
X-ray Astrophysics, in the focus of grazing incidence telescopes, it can
perform angular resolved polarimetry with a huge improvement of sensitivity,
when compared with the conventional techniques of Bragg diffraction at 45
degrees and Compton scattering around 90 degrees. This configuration is the
basis of POLARIX and HXMT, two pathfinder missions, and is included in the
baseline design of IXO, the very large X-ray telescope under study by NASA, ESA
and JAXA.
