User's Guide – 2.3 Event-based astrometric correction
XMM-Newton source coordinates include a generally small systematic error arising from small offsets between the nominal telescope pointing and the actual boresight. In the catalogues from individual observations, they are reduced by comparing the X-ray with optical and infrared source positions in the SDSS, USNO-B1.0, and 2MASS catalogues. Field shifts in RA and Dec and field rotation are derived for each observation after performing source detection.
When several overlapping observations are processed, which have their individual offsets, corrections have to be applied before source detection. An event-based astrometric field correction was thus introduced with 4XMM-DR9s. During pre-processing, the recorded events are shifted to corrected positions. The corrections are taken from the results of the 4XMM-DR9 field rectification. The images on which stacked source detection is performed are created from the corrected attitude files and event lists, resulting in higher positional accuracy of the fit (cf. Traulsen et al. 2020). For 4XMM-DR9 observations that could not be corrected – e.g. because of a lack of optical and infrared counterparts –, a mean systematic uncertainty of about 1.3" remains (cf. Webb et al. 2020).