XMM-Newton Science Analysis System


eootepileupmask (ebkgmap-2.9) [xmmsas_20160201_1833-15.0.0]

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Out-of-time events and pile-ups

Out-of-time (OOT) events are associated with any CCD instrument. In CCD instruments, events in each pixel are read by transferring those laterally from pixel to pixel (i.e., frame-transfer) before they reach either the read-out node or storing region, which has no sensitivity for photons in the energy band of interest.

In most of the cases, this transfer (into a reading region) takes much shorter time than the actual exposure. However it still may take a significant time. If there is a bright enough source(s), a significant number of the photons from the source reach pixels during this frame-transfer stage. These happen in the following procedure. Suppose there is a single (bright) source with a flux of $R$ [count/frame] (in the no-pileup situation) at a Y-axis position of $y=y_{\rm S}$, where the frame transfer is carried out in the direction of negative Y-axis (towards $y=0$) and the Y-axis in the imaging area has $y_{\rm Max}$ pixel. This frame is transferred into the reading (non-exposed) area, which takes a time of $t_{\rm F}$. Now a frame in the imaging area on the CCD is exposed for a given exposure, $t_{\rm E}$. It is then transferred to the reading area. During this transfer pixels at Y-axis positions between $y_{\rm S} < y < y_{\rm Max}$ are inevitably exposed to the source with the total exposure of

$$\frac{t_{\rm F}}{t_{\rm E}} \times \frac{y_{\rm Max}-t_{\rm S}}{y_{\rm Max}}.$$ (1)

In this frame transfer, new blank pixels are also transferred to the imaging area for the next exposure. In this transfer the pixels at Y-axis positions between $0 < y < y_{\rm S}$ are inevitably exposed to the source with the total exposure of

$$\frac{t_{\rm F}}{t_{\rm E}} \times \frac{t_{\rm S}}{y_{\rm Max}}.$$ (2)

Consequently all the pixels along the row are affected by the OOT events with the total rate of count per frame of

$$R\times \frac{t_{\rm F}}{t_{\rm E}}.$$ (3)

In the standard source detection scheme for EPIC in SAS (emldetect), this effect is taken into account, based on the detected count rates. However if a source is too bright and causes a significant pile-up, then the count rate of the source ($R$ in the above notation) is underestimated, because the current source detection scheme works on the basis that there is no pile-up. As a result any estimate of parameters of the sources which are located at (or very close to) an OOT event streak is likely to be wrong.

This task creates a mask image, which represents the OOT event streaks affected by significant pile-ups, if foroote=`yes'.