A sealed Gas Pixel Detector for X-ray astronomy

We report on the results of a new, sealed Gas Pixel Detector. The very compact design and the absence of the gas flow system make this detector substantially ready for use as focal plane detector for future X-ray space telescopes. The instrument 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 that are imaged by a high-gain (>1000), fine pitch GEM that matches the pitch of a pixel ASIC which is the collecting anode of the GPD (105k, 50 μm wide, hexagonal cells). The device is able to simultaneously perform good imaging (50–60 μm), moderate spectroscopy (∼15% at 6 keV) as well as fast, high-rate timing in the 1–10 keV range. 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 percent level will be possible, making for a real breakthrough in high-energy astrophysics.