Photoelectric X-ray Polarimetry with Gas Pixel Detectors

The Gas Pixel Detector, recently developed and continuously improved by Pisa-INFN in collaboration with IASF-Roma (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 polarization plane of X-Ray photons, while preserving the information on the absorption point, the energy and the time of arrival of individual photons. Applied to X-ray Astrophysics, in the focus of grazing incidence telescopes, it can perform angular and energy resolved polarimetry with a large improvement of sensitivity, when compared with the conventional techniques of Bragg diffraction at 45° and Compton scattering around 90°. This configuration has been the basis of POLARIX and HXMT, two pathfinder missions, and was included in the baseline design of IXO, the very large X-ray telescope under study by NASA, ESA and JAXA. We have recently improved the design of this low energy polarimeter (2–10 keV) by modifying the geometry of the absorption cell to minimize any systematic effect that could leave a residual polarization signal for non-polarized source. We will report on the testing of this new concept.