Pile-up occurs when a source is so bright that there is the non-negligible possibility that two or more X-ray photons deposit charge packets in a single pixel (``photon pile-up''), or in neighboring pixels (``pattern pile-up'', i.e. singles pileup to doubles etc.), during one read-out cycle (i.e. one frame). In such a case these events are recognized as one single event having the sum of their energies. If this happens sufficiently often, this will result in a hardening of the spectrum as piled-up soft events are shifted in the spectrum to higher energies.
In addition, pile-up leads to a more or less pronounced depression of counts in the central part of a bright source, resulting in flux loss. Pile-up also affects light curves, suppressing high count rates.
The XMM-Newton UHB lists (readout mode dependent) maximum count rates above which a source suffers from pile-up. In general the MOS camera is much more susceptible to pile-up than the pn.
To check whether pile up indeed is a problem, use the SAS task epatplot. (see the thread at:
http://xmm.esac.esa.int/sas/current/documentation/threads/epatplot.html)
To run epatplot one needs to create an event file for the source as described below in step 3) of § 4.10.1. The input event file name (e.g., src_evlist.fits) must be specified via the epatplot task set parameter. If the resulting plot shows the model distributions for single and double events diverging significantly from the measured distributions, this is a strong indication that pile-up has occurred. Fig. 16 shows an example of a bright source observed in pn full-frame mode which is strongly affected by pile-up. Due to ``pattern pile-up'' more doubles are produced at the expense of single pixel events.
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A common strategy adopted to analyse spectra of piled up sources has been to excise the core of the point-spread function (PSF). Recent investigations have shown that, at least in some cases, this strategy can yield incorrect results. The problem is most likely related to an inaccurate modeling of the energy dependence in the PSF model used by arfgen to evaluate encircled energy fractions.
The user who suspects that pile-up may be affecting his/her observation
is advised to first use the epatplot tool to asses the presence and
level of pile-up. In case of moderate pile-up (see XMM
Newton Users Handbook,
[1]) the user is advised
to make use of pattern 0 spectra, which are less sensitive than other patterns,
for both MOS and pn cameras and use the new method for pile-up correction
provided by S. Molendi and S. Sembay (2003) [4].
The older method, of excising the inner part of the source emission from the event list used for the creation of the spectrum, is achieved via the xmmselect task, first displaying the whole source region as an image (see § 4.9) and then defining an annulus (via the ds9:Region menu and importing the region into the xmmselect selection expression via the ``2D region'' button (see § 4.10.1, step 5)). With this selection expression, a filtered event list, named e.g. src_annulus_evlist.fits, can be created with xmmselect. Finally epatplot should be called again now with the src_annulus_evlist.fits as input data set. Inspecting the created pattern distribution curves, the inner radius of the annulus should be increased as long as the pattern distribution agrees with the model. Note, excluding the inner part of the source from the analysis will of course reduce the number of events for further analysis. So an iterative process for finding the best exclusion inner radius should be performed. Fig. 17 shows the pattern distribution of the same source as above, but after exclusion of the inner part of the source. The pattern distributions now agree with the model curves and a resulting spectrum would in principle be free of pile-up effects.
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