Outcome of the IXPE instrument calibration

IXPE (Imaging X-ray Polarimetry Explorer) is the next NASA Small Explorer mission foreseen for the launch in 2021. It is a partnership with the Italian Space Agency (ASI). IXPE is devoted to X-ray polarimetry in the 2-8 keV energy band. The IXPE telescope comprises three grazing incidence mirror modules coupled to three Detector Units (DUs) hosting each one a Gas Pixel Detector (GPD) polarimeter. The GPD, developed by the INFN and INAF-IAPS Italian research institutes, exploits the photoelectric effect to measure the linear polarization of the X-ray emission from astrophysical sources. Its spectroscopic (energy resolution < 20% at 6 keV), timing and imaging capabilities allow IXPE to perform unprecedented polarimetric measurements with high significance. A wide and accurate on ground calibration was carried out on the IXPE detector units at INAF-IAPS in Rome. A dedicated facility was set-up to calibrate the DUs with polarized and unpolarized X-rays at different energies in the operating energy band. In this proceeding we introduce this calibration set-up called Instrument Calibration Equipment (ICE) and the relevant outcomes from the calibration activity that confirms the capability of IXPE to open the window of high significance polarimetry in X-rays.