serial charge transfer inefficiency modelling and radiation damage study

During the course of its mission, ESA’s Gaia spacecraft has generated a map of the stars of the Galaxy of exquisite detail. While in its L2 orbit, the satellite has been exposed to high-energy cosmic rays and solar particles, which caused permanent damage to its CCDs. The main effect of radiation damage on Gaia data is the distortion of its images and spectra that is caused by the charge transfer inefficiency (CTI) of the CCDs during the readout process. When this is not taken into account, it can result in inaccurate measurements of a star’s location and flux. We analysed and modelled the effect of CTI on the serial readout direction, which is larger than in the parallel due to the presence of CCD manufacturing defects. A pixel-based physically motivated CTI model, <ns0:monospace>CtiPixel</ns0:monospace>, was developed to characterise the damage in Gaia CCDs. The model was calibrated using dedicated serial CTI diagnostic data, taken every three to four months over the course of the mission. The model is shown to be a good representation of the observed signatures of CTI in the calibration datasets, and its parameters reveal significant insights into the nature of the CCD defects generated by space irradiation. The damage in the serial direction slightly increases linearly over time in general, with sudden step changes after strong solar flares and coronal mass ejections directed towards Earth. The serial CTI showed a further step increase as a consequence of the engineering CCD annealing experiment that was carried out after the completion of Gaia science observations.