Time-dependent instrumental effects in IXPE: Pressure variation and GEM charging inside GPDs

We present the results of two studies conducted on long-term effects of a Gas Pixel Detector (GPD), central point of the X-ray detection of the Imaging X-ray Polarimetry Explorer (IXPE) mission: the pressure variation inside the sealed gas cells of the instrument and the charge build-up in the dielectric layer of the Gas Electron Multiplier (GEM), used as the amplification stage of the detector.