While optical polarization of astronomical sources has already been studied extensively, only a single source has been observed in X-rays so far: the Crab nebula. This well-known object is the remnant of the explosion of a star in a supernova which was observed by Chinese astronomers in the year 1054. It contains a pulsar in its center which is creating a pulsar wind nebula. In the seventies, the X-ray polarization of the source was observed for the first time by several independent missions. The measurements revealed a degree of polarization of about 20%. To date, only limits are known for the degree of polarization of other sources.
XIPE would be launched 8-10 years after funding was provided and it would grant the ability to resolve X-ray polarization in time, space and spectrally and will thus dramatically improve the sensitivity of X-ray polarization measurements. After 40 years without further X-ray polarimetry measurements, XIPE is now expected to increase the number of X-ray sources with known polarization from one to several hundreds.