Users should be aware of the different methods of processing events that are detected in the EPIC MOS and pn cameras. Event selection is performed on-board in order to allow transmission of useful data only (i.e., removal of empty pixels from the data stream). Also a significant number of X-ray events are expected to be split between pixels, and these must be recognised as such.
The MOS camera uses an event pattern recognition scheme. All detected events are transmitted to ground with a pattern identification (similar to the ASCA ``grades'') and a number of energy data. The Science Analysis Subsystem (SAS) software will automatically allow these data to be reconstructed to a single energy value in an event list. However, the user will be able to select sub-sets of pattern types because optimal energy resolution may be obtained from a sub-set of events (one and two pixels in size). If maximum source detection efficiency is required all patterns might be selected. Therefore, users must choose carefully the dataset for the science of interest, and further beware that not all pattern combinations may be calibrated as well as some default sets.
The pn camera conversely telemeters all pixels above a certain threshold, and the X-ray event selection and processing is performed wholly on-ground, e.g., through the use of the SAS. However, we emphasise that default processing should be followed, because the details of events carried in the event list are crucial to performing proper gain calibration for example.
A host of secondary effects of event processing is foreseen (for example the amount of pile-up in the PSF may depend on event pattern selection, particle background rejection is heavily dependent on event selection criteria), but the scope may not fully be appreciated until further reduction of ground calibrations is performed by the instrument PI teams, or after in-orbit experience accumulates.