X-ray polarimetry with Gas Pixel Detectors: A new window on the X-ray sky

We report on a new instrument that brings high sensitivity to X-ray polarimetry, which is the last unexplored field of X-ray astronomy. It derives the polarization information from the track of the photoelectrons imaged by a finely subdivided Gas Pixel Detector. The device can also do simultaneously good imaging (50–100 μm), moderate spectroscopy (∼15% at 6 keV) and fast, high rate timing down to 150 eV. Moreover, being truly 2D, it is non-dispersive and does not require any rotation. The great improvement of sensitivity, at least two orders of magnitude with respect to traditional polarimeters (based on Bragg crystals or Thomson scattering), will allow the direct exploration of the most dramatic objects of the X-ray sky. At the focus of the large mirror area of the XEUS telescope, it will be decisive in reaching many of the scientific goals of the mission. With integration times of the order of 1 day polarimetry of Active Galactic Nuclei at the per cent level will be possible, a real breakthrough in high-energy astrophysics.