  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 micron wide, hexagonal cells). The device
is able to simultaneously perform good imaging (50-60 micron), moderate
spectroscopy (~15% at 6 keV) as well as fast, high rate timing in the 1-10keV
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 one day, polarimetry of Active
Galactic Nuclei at the per cent level will be possible, making for a real
breakthrough in high energy astrophysics.
