XIPE is a new mission concept selected by ESA in June 2015 to undergo a 2 years-long assessment phase in the context of the Cosmic Vision M4 competition. The mission is devoted to the observation of celestial sources in X-rays:
X-ray polarimetry is photon hungry, but scientifically needed
A dedicated mission is required
XIPE has not been selected as the ESA M4 mission candidate to be implemented, but a similar mission is being developed by NASA and it is named IXPE.