A pixel-by-pixel equalization method for the X-ray imaging polarimeter on board the IXPE mission

On December 9, 2021, the Imaging X-ray Polarimetry Explorer (IXPE) observatory was launched, carrying three X-ray polarimeters based upon the Gas Pixel Detector (GPD). These devices measure the photoelectron's ionization track following absorption of an X-ray, from which the photoelectron's initial direction (correlated to the polarization position angle) is determined. Here we describe a method for event-by-event correction of pixel-by-pixel gain variations, which are found to be +/-20%, by comparing the charge in each pixel with the average at the same relative position inside tracks of the same shape. Using the large dataset acquired during on-ground calibration of the IXPE detectors, we have individually calibrated each of the 300x352 pixels of each detector's ASIC. We discuss the performance improvements obtained using this method, which may be relevant to other instruments that detect individual events through images.