XMM-Newton Data Analysis

Description valid for XMMSAS version 5.3
If a photon hits a pixel of the semiconductor CCD chip it releases a number of electron-hole pairs depending on the energy of the incoming photon. Some part of this charge cloud may be deposit into a neighbouring pixel. This can happen when a) the photon hits the CCD close to the edge of the pixel, or b) at higher energies the released electrons diffuse into the neighbouring pixel. In these cases we speak of Doubles, Triples and Quadruples, depending on how many pixels are involved (see the descriptions of the XMMSAS tasks epevents for the EPIC PN and emevents for the EPIC MOS). As mentioned before the fraction of Singles, Doubles, Triples and so on depends on the energy of the penetrating photon. In order to display the fraction of single, doubles, etc of your source photons the XMMSAS offers the task epatplot. This task also helps you to determine if your source has been in pile-up or not.

Please perform the following steps:
Select the region of your source from your input event file as described on Select region page.
Create a new event file that only contains the source files as described on Filtered event file page. This output event file is used to display the singles, doubles, etc. in the source:
 evselect table=events.fits withfilteredset=true destruct=yes \
 	  keepfilteroutput=true \
	  expression="circle(25600,27900,500,X,Y)" \
     	  filteredset=circle.fits
Now call the the XMMSAS task epatplot. It will create a postscript output file called ..._pat.ps:
    epatplot set=circle.fits
The output postscript file will have the default name circle_pat.ps.

Display of the singles, double, etc.

The upper panel displays the spectra of Singles (red) , doubles (blue), triples (green), and quadruples (light blue)

The lower panel displays the fraction of the single, double, triple, and quadruple pattern to all events in the event file. The color codes are the same as given above. We can see that the fraction of single events is decreasing with increasing energy. The solid coloured lines display the expected fraction from model curves. For the EPIC PN models exist for all modes except for the burst mode. For EPIC MOS only a common model is available for MOS1 and 2 for all modes.
The XMMSAS task epatplot is a powerful tool to check whether your source had been in pile-up during the observation. The example shows a EPIC-PN Small-Window observation of PKS 2155-304:

Display of the singles, double, etc.

As above, the spectra of Singles (red) , doubles (blue), triples (green), and quadruples (light blue)

We can clearly see that there is an excess of double events between about 0.7 - 4.0 keV compared to the model. This is a clear sign of pile-ups.

In order to avoid pile-up, do not take the photons of the center of the source. Instead select the photons in a ring to avoid the region where pile-ups occur. The image below displays the photons selected in a ring that avoids the central region of the source. Here, the observed data follow the predicted model at higher energies.

Display of the singles, double, etc.