The SAS calculates the initial energy of the photon arriving in a CCD,
using a large number of corrections based on temporal, spatial and
energy information. Nevertheless, in some cases the photon energies
can still be seen to be shifted from their true positions, when modelling
the spectrum in a spectral fitting package. Principally this becomes
obvious by residuals which are visible around the Si 
line at 1.84 keV and the Au-M edge at 2.2 keV and AU-L edge at 11.9 keV.
eveventshift provides the user with an ad hoc method of moving the energies to remove these residuals.
The original event energies are stored in a new column called PI_ORIG.
This task is currently restricted to EPIC-pn event files, taken from observations made with the FastTiming or FastBurst modes. The header keyword SUBMODE must contain one of those strings for the task to run.
