The Gas Pixel Detectors (GPD) rely on the anisotropy of the emission direction of photoelectrons produced by polarized photons to gauge with high sensitivity the polarization state of X-rays interacting in a gaseous medium. Position-dependent and energy-dependent polarization maps of such synchrotron-emitting sources will reveal the magnetic-field structure of the X-ray emitting regions. X-ray polarimetric imaging better indicates the magnetic structure in regions of strong electron acceleration. The system is capable to resolve point sources from surrounding nebular emission or from adjacent point sources.
