Reconstruction of X-ray event energies requires a knowledge of the
level of dark-current present in each CCD pixel. This is estimated
on-board, for the EPIC-pn camera![[*]](../icons/footnote.png){kind=icon}
, by making a special reading of the
first 50 or 100 exposed frames. An offset map is then calculated by
taking the average charge in each pixel, after discarding the highest
three and lowest three charges. The resulting map is important for
removing the effect of optical loading, where many optical photons can
bias upwards the apparent energies of X-ray photons.
A problem occurs when the X-ray source is bright and, for a given pixel, more than three of the first 50 (or 100) frames contain X-ray photons. This leads to an effect known as X-ray loading where the dark-current is over-subtracted from events falling in affected pixels and hence energies appear artifically low. The flux level where this becomes important is roughly the same as that when pile-up effects occur.
This task, epxrlcorr, attempts to correct event energies by statistically estimating deviations in the offset map caused by X-ray loading. For a more complete description of X-ray loading in the EPIC-pn camera please see XMM-SOC-CAL-TN-0050.
