in the number density of the A-type manufacturing species, that dominated the defects at
Article number, page 11 this surprising result has been attributed to the sudden temperature variation in the Gaia focal plane recorded in May 2024, that contributed to the changes in measured CTI.

[Figure: Fig. 12: Scatter plot of trap capture cross–sections [cm2] versus energy level [eV] for the 3 trap–species model fits to the entire Gaia mission data set. The red, green and blue dots are relative to the first, second and third trap species in the CtiPixel model fit.]

The colder operating temperatures have reduced its impact in the parallel direction, while in the serial register the readout losses have increased significantly. At this cooler setting the traps emission timescale have shifted to longer values (see Equation 5), impacting in opposite ways the ability of traps to capture electrons in the image section and the readout register, due to their different readout transfer frequencies. Figure 14 shows fits parameters resulting from the fits of the entire set of serial calibration data available during the mission in the form of scatter plots and histograms. Outlying points from fits with Goodness of Fit (GoF) on the tail of the distribution are not shown in the Figure. In particular, the (ROW3, AF9) CCD is not well modelled by CtiPixel. This device presents a defect in column AC=1977, that could be responsible for the excess charge in the first post-scan pixel due to deferred charge that is causing poor fits of the charge trails, especially at high charge injection levels. We notice that (ROW3, AF9) is a ’twin’ CCD, one of 10 pairs of devices present in the Gaia focal plane (we use the term ’twins’ to indicate a pair of CCDs manufactured from the same silicon wafer; the (ROW3, AF9) pair is (ROW2, AF4), whose fit presents a typical GoF value). Appendix D in Rowell & Davidson (2026) presents a list of Gaia CCD twins and information on the device types. With respect to β, the charge volume coefficient, the parameter that in CtiPixel links the number of signal electrons with the confinement volume (the physical volume filled by the charge packet in a pixel), the measured distribution for the third trap species appears to separate from the other types of defects. This result could indicate a different physical distribution of this trap species within the pixels, and could indeed suggest that these defects are still migrating in the lattice and have not yet bonded into a stable defect. Another interesting result to note in these plots is the different parameter space occupied by the red variant (RP) devices with respect to the blue CCDs, in terms of both the energy levels (and hence release timescale) and the traps cross sections. This result is not well understood. As shown in Figure 8, the number

[Figure: Fig. 10: Studentised (unit–weight error) residuals as a function of trail position for all 3–trap species calibrations: (upper panel) blue variant (AF+BP) devices; (lower panel) red variant (RP) devices. Thick dashed lines indicate 16%, 50%, 84% (red, black, red) quantiles respectively corresponding to distribution median ±1σ. The thin red lines show the ±1 expected position assuming normally–distributed uncertainties.]

measured CTI in the image section (parallel readout direction), while in the serial register it resulted in a modest step increase of the total trap density. The opposite appears to be true after the 2024 period of strong solar activity. On further investigation,

[Figure: Fig. 11: Distribution of trap release timescale values from the final 3–trap species calibrations, calculated from the best fit trap energy values of the calibration activity at epoch OBMT = 8224 revolutions. Three well defined peaks for the different species of the CtiPixel model are present, including the fastest manufacturing species that dominated the CCD defects at launch and two slower radiation-induced traps. A tail of slower emission times can be observed for the third species. These can be interpreted as emerging defects still in an unstable configuration, similarly to the radiation-induced defects measured on board the Euclid mission CCDs.]

Article number, page 12

[Figure: Fig. 13: Evolution of the total density of traps for the CCD in row 3 and AF strip 6. The observed trend is caused by a gradual increase in damage caused by cosmic rays and sudden step jumps in correspondence to solar flares event, shown as the red lines for the class X8.2 flare of September 2017 and as the start of the intense period of solar activity of Spring-Summer 2024.]