4.4.1 Filtering EPIC MOS concatenated event lists

Each of the two EPIC MOS cameras consists of seven individual 600 x 600 pixel CCDs. Because of telemetry constraints, a real time on-board recognition scheme filters out cosmic-ray tracks and exclusively transmits to the ground the information supposedly related to X-ray events. The on-board recognition scheme looks for a local enhancement of signal in flat fielded images. The signal enhancement is searched in 5 x 5 pixel matrix which is scanned over the full image. The signal is defined with respect to a threshold value set by telecommand for each observation. An event is identified, if in the 5 x 5 pixel matrix, pixels above thresholds formed a predefined pattern.

In case of imaging mode, 32 patterns have been predefined (see Fig. 12, upper panel). They each correspond to an isolated event i.e. to a zone above threshold completely encircled by pixels below threshold. There are however two exceptions. Pattern 30 can be connected to a pixel above threshold on the diagonal of the center pixel. Pattern 31 can have any of the border pixels above threshold. Pattern 30 and pattern 31 are designed to quantify the amount of cosmic-rays extended tracks.

**Figure 12:** List of EPIC-MOS patterns: the *upper panel* for **imaging mode** should be interpreted as follows: each pattern is included in a 5 x 5 matrix used for proximity analysis, a pattern is centered by definition on the pixel with highest charge, this central pixel is colored in red, the other pixels above threshold in the pattern are colored in green, all pixels colored in white must be below threshold, the crossed pixels are indifferent (they can be above threshold). The philosophy for patterns 0-25 is that a good X-ray pattern must be compact, with the highest charge at the center, and isolated (all pixels around are below threshold). Patterns 26-29 are the so-called diagonal patterns, not expected from a genuine X-ray, but which can arise in case of Si-fluorescence or of pileup of two monopixel events. The *lower panel* for **timing mode** should be interpreted in the same way as in imaging mode, with the difference that the place where maximum charge occurs is ignored. Due to this, all doubles appear as Pattern 1, whether leading or trailing. Patterns 2 and 3 are mostly not due to true X-rays, but to cosmic-ray tracks.
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On-ground calibration of the imaging mode has shown that soft X-rays mainly generate patterns 0 to 12 corresponding to compact regions of X-ray energy deposition. Pattern 0 events are single pixel events. These comprise most of the valid X-ray events with the most accurate energy resolution. For imaging mode data patterns 0 to 12 are the canonical set of valid X-ray events which are well calibrated. Selection of these patterns constitutes the best trade-off between detection efficiency and spectral resolution. However, because they deposit energy below the CCD depletion zone, higher energy X-rays also generate pattern 31 events with a probability of 20% and 50% respectively at 6 keV and 9 keV. Pattern 31 comprises largely cosmic ray events but can also include pile-up X-ray events. A large density of pattern 30, 31 events (and 26

29 diagonal events) in the core of the telescope point spread function (PSF) is the signature of a piled response which needs careful analysis (see § 4.7).

In case of timing mode data, the pattern analysis is purely 1-dimensional (i.e. insensitive to other rows, see Fig. 12, lower panel), because each timing `row' is actually the sum of 100 true rows, so the rows are not physically related.

The users are encouraged to test different filtering schemes which could be better suited to their own observation. However, general recommendations are as follows (for further details see XMM-SOC-CAL-TN-0018[7], a document describing the current status of EPIC calibration and data analysis):

XMM-Newton Science Analysis System: Users' Guide

4.4.1 Filtering EPIC MOS concatenated event lists