  Polarimetry is widely considered a powerful observational technique in X-ray
astronomy, useful to enhance our understanding of the emission mechanisms,
geometry and magnetic field arrangement of many compact objects. However, the
lack of suitable sensitive instrumentation in the X-ray energy band has been
the limiting factor for its development in the last three decades. Up to now,
polarization measurements have been made exclusively with Bragg diffraction at
45 degrees or Compton scattering at 90 degrees and the only unambiguous
detection of X-ray polarization has been obtained for one of the brightest
object in the X-ray sky, the Crab Nebula. Only recently, with the development
of a new class of high sensitivity imaging detectors, the possibility to
exploit the photoemission process to measure the photon polarization has become
a reality. We will report on the performance of an imaging X-ray polarimeter
based on photoelectric effect. The device derives the polarization information
from the track of the photoelectrons imaged by a finely subdivided Gas Pixel
Detector. It has a great sensitivity even with telescopes of modest area and
can perform simultaneously good imaging, moderate spectroscopy and high rate
timing. Being truly 2D it is non-dispersive and does not require any rotation.
This device is included in the scientific payload of many proposals of
satellite mission which have the potential to unveil polarimetry also in X-rays
in a few years.
