XMM-Newton Users Handbook

3.3.10 EPIC out-of-time events

For the EPIC imaging modes, photons are not only registered during the actual integration interval but also during the readout of the CCD (shift of charges along a column towards the readout node). These so called Out-of-Time (OoT) events get a wrong RAWY value assigned and thus finally a wrong CTI correction. The magnitude of this effect scales with the ratio of readout time and integration time. Table 6 summarises the relevant parameters.

Table 6: EPIC Out-of-Time (OoT) events. The percentage of OoT events is given by the mode dependent ratio of readout time and integration time
MOS Mode Life time with OoT events [%] Fraction OoT [%]
Full frame 100.0 0.35
Large window 99.5 0.5
Small window 97.5 0.5
pn Mode Life time with OoT events [%] Fraction OoT [%]
Full frame 99.9 6.3
Ext. full frame 100.0 2.3
Large window 94.9 0.16
Small window 71.0 1.1

The effect of OoT events broadens spectral features in a systematic way: assume a bright point-like source above CCD pixel RAWY_S then the OoT events with RAWY $<$ RAWY_S will be undercorrected (and thus get a too low energy) while the ones for RAWY $>$ RAWY_S will become overcorrected (too high energy).

This effect is illustrated in Fig. 41 using the internal calibration source (CalClosed filter position) as a quasi example of a bright extended source with a line spectrum. Note the significantly broadened features in the OoT spectrum.

Figure 41: Effect of spectral broadening due to OoT events: the upper panel shows a spectrum extracted from a pn CalClosed full frame mode observation (black) together with the spectrum corrected for OoT events (green). In the lower panel the spectrum of the simulated OoT events over the whole FoV is plotted.
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The effect on images is shown in Fig. 42 (also note the arc-like structures due to single mirror reflections at the top left of the FoV; see § 3.2.4).

Figure 42: Effect of OoT events on images: The upper left panel contains a 2-10 keV band image of a pn observation of a bright source in full frame mode with the OoT-events visible as a strip running from the source toward the top of the image (in detector coordinates). The upper right panel depicts the modelled (see SAS task epchain) OoT event distribution where in the lower left panel these are subtracted from the original image. The lower right panel is cleaned for the soft 0.2-2 keV band for comparison.
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The OoT events are not only affecting the central source itself but also the region between the source and the readout node. In Fig. 42 for example a source is located close to the upper edge just along the readout direction of the bright central source. Also the spatial and spectral analysis of any extended emission (as visible e.g. in the lower right panel of Fig. 42) can significantly be hampered by OoT events.

If a performed EPIC-pn observation is affected by OoT events, the SAS task epchain can be used to create a simulated OoT events list. Images and spectra extracted from this OoT events list should be subtracted from images and spectra produced from the original events list to clean these products from the effects of OoT events.

In Fig. 43 we show an example on how the EPIC-pn camera works in Small Window mode. The process is as follows:

  1. Immediately after a readout is completed the top 64 rows of the CCD (the ones downlinked to the ground) are fast shifted downwards by 64 rows. This takes 0.046 ms (64x720 ns).
  2. A nominal integration of 4.028 ms follows.
  3. There is another fast shift of the top 64 rows but now they are moved to the readout node (i.e. they are shifted 136 rows). This takes 0.098 ms (136x720 ms).
  4. The 64 rows of the science window are finally readout. This takes 1.518 ms (66x0.023 ms, as two additional lines are read to start the pipeline).
  5. Then, the contribution of out-of-time events to the background within the science window can be divided in three parts:

Figure 43: Illustration on how the EPIC-pn camera in Small Window mode works. For clarity the grid is 4x4 pixels. Left panel is the input sky image. Right panel shows the out-of-time events in the science window region. See text for details.
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European Space Agency - XMM-Newton Science Operations Centre