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Events are anything recorded by the CCD. They could be many things ranging from X-ray photons, charged particals, or noisy electronics, just to name a few. The science is to determine what you are interested in and how to identify them. We hope that most of the events detected are true X-ray photon events and this is for the most part true.
Before events are extracted, each raw FITS image is bias and overclock corrected. This is done in three steps.
where n(q) is the number of overclock pixels in each quadrant
These bias corrected pixel values are then used for event detection and subsequent extraction. Note that the reason for the overclock pixels is to allow removal of short term drift in the electonic baseline on the timescale of CCD row readout.
Depending on the flux of the source, events can be extracted in one of two ways: photon counting mode and integration mode. Both of these are described in detail below.