  The NASA/ASI Imaging X-ray Polarimetry Explorer, which will be launched in
2021, will be the first instrument to perform spatially resolved X-ray
polarimetry on several astronomical sources in the 2-8 keV energy band. These
measurements are made possible owing to the use of a gas pixel detector (GPD)
at the focus of three X-ray telescopes. The GPD allows simultaneous
measurements of the interaction point, energy, arrival time, and polarization
angle of detected X-ray photons. The increase in sensitivity, achieved 40 years
ago, for imaging and spectroscopy with the Einstein satellite will thus be
extended to X-ray polarimetry for the first time. The characteristics of gas
multiplication detectors are subject to changes over time. Because the GPD is a
novel instrument, it is particularly important to verify its performance and
stability during its mission lifetime. For this purpose, the spacecraft hosts a
filter and calibration set (FCS), which includes both polarized and unpolarized
calibration sources for performing in-flight calibration of the instruments. In
this study, we present the design of the flight models of the FCS and the first
measurements obtained using silicon drift detectors and CCD cameras, as well as
those obtained in thermal vacuum with the flight units of the GPD. We show that
the calibration sources successfully assess and verify the functionality of the
GPD and validate its scientific results in orbit; this improves our knowledge
of the behavior of these detectors in X-ray polarimetry.
