  X-ray polarimetry, sometimes alone, and sometimes coupled to spectral and
temporal variability measurements and to imaging, allows a wealth of physical
phenomena in astrophysics to be studied. X-ray polarimetry investigates the
acceleration process, for example, including those typical of magnetic
reconnection in solar flares, but also emission in the strong magnetic fields
of neutron stars and white dwarfs. It detects scattering in asymmetric
structures such as accretion disks and columns, and in the so-called molecular
torus and ionization cones. In addition, it allows fundamental physics in
regimes of gravity and of magnetic field intensity not accessible to
experiments on the Earth to be probed. Finally, models that describe
fundamental interactions (e.g. quantum gravity and the extension of the
Standard Model) can be tested. We describe in this paper the X-ray Imaging
Polarimetry Explorer (XIPE), proposed in June 2012 to the first ESA call for a
small mission with a launch in 2017 but not selected. XIPE is composed of two
out of the three existing JET-X telescopes with two Gas Pixel Detectors (GPD)
filled with a He-DME mixture at their focus and two additional GPDs filled with
pressurized Ar-DME facing the sun. The Minimum Detectable Polarization is 14 %
at 1 mCrab in 10E5 s (2-10 keV) and 0.6 % for an X10 class flare. The Half
Energy Width, measured at PANTER X-ray test facility (MPE, Germany) with JET-X
optics is 24 arcsec. XIPE takes advantage of a low-earth equatorial orbit with
Malindi as down-link station and of a Mission Operation Center (MOC) at INPE
(Brazil).
