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The emission of X-Rays from most of astronomical objects starts with not thermal processes and is modified by transfer in extreme magnetic or gravitational fields and scattering in plasmas with asymmetric geometries. The degree of polarization expected is much higher than in other energy bands. After the first attempts more than 40 years ago the limited sensitivity and the difficulty to control systematics have discouraged the implementation of such experiments.
The Imaging X-Ray Polarimetry Explorer (IXPE), a Small Mission by NASA with a substantial contribution of ASI, INAF and INFN, should be launched within one year. Based on the visualization of photoelectron tracks in a focal plane gas detector, IXPE will give a first view of this subtopics of X-Ray Astronomy by measuring the polarization of tens of sources (50 – 55 in the first year) belonging to most classes of the X-Ray Sky.
Polarimetry will help to better understand the mechanism of emission and transfer of radiation. It should be particularly effective in disentangling the effects of physics from those of geometry. I give a few examples including the measurement of spin in galactic black holes, the structure of magnetic fields in shocks in Supernovae Remnants and in Pulsar Wind Nebulae, the site of seed photons in comptonized bumps in blazars. I also mention which measurement relevant for fundamental physics have been suggested.
Eventually I will give some hint about the future developments of experiments aimed to extend this technique to investigations not covered by IXPE for the sensitivity or for the energy band.