XMM Examples - Handling of Pile-up

Pile-ups appear in the EPIC MOS and PN detectors when more than one photon hits the same CCD pixel during the integration time of a frame (e.g. 68.702 ms for the EPIC PN in Full-Frame mode) and deposit their energies into the detector. The result is that we are unable to tell if there were several photons detected during this time or only one with the energy equivalent to several photons. As a result of pile-ups the read out electronic registers only one photon but with the energy of all involved photons. Therefore the X-ray spectrum of a source looks harder than it actually is. The longer the integration time and the brighter the source the larger is the chance to get pile-ups in your observation. If you are just planning an observation, you should use an observing mode that suits your source the best, e.g. it does not make sense to observe a bright source with e.g. 20 cts/s with the PN in Full-Frame mode. Use the Small-Window or even Timing Modes instead. The way to check this before your observation is by estimating the pile-up fraction by the mission Count Rate Simulator WebPIMMS at Goddard.
If you already use an observation that is affected by pile-ups, please follow the following steps to extract a usefull spectrum of the source:

First step is to check whether your source is in pile-up or not. The way to check this is by the XMMSAS task epatplot as described on the Single-Double page.

If the plot produced by epatplot for your source does not agree with the model as shown in the example on the Single-Double page, the next step is to leave the inner region of the source out and use only the photons in a ring of the outer region of the source.

To exclude the inner region of the source, please choose sky pixel or detector pixel. Do not use RAWX, RAWY raw CCD pixels, especially only one single RAW pixel. The reason is that in this case you take out charges from double events and you will get a different, wrong single/double fraction.

Now you will be able to perform a spectral analysis which will give you the correct spectral shape of your source spectrum. Please keep in mind that the fluxes are wrong of course and have to be corrected by the photons that have been left out from the central region of the source.

Please note: Pile-ups can also appear in fainter sources if the background is high or the source is located on the detector in a region that is noisy, e.g. close to the CAMEX.