Depending on how bright the source is and what modes the EPIC detectors are in, event pile up may be a problem. Pile up occurs when a source is so bright that incoming X-rays strike two neighboring pixels or the same pixel in the CCD more than once in a read-out cycle. In such cases the energies of the two events are in effect added together to form one event. If this happens sufficiently often it will skew the spectrum to higher energies. To check whether pile up may be a problem, use the SAS task epatplot. Note that this procedure requires as input the event files created when the spectrum was made.
The output of epatplot is a postscript file, which when downloaded to the user's machine, may be viewed with gv. It contains two graphs describing the distribution of counts as a function of PI channel; see Figure 7.4. The top graph is the distribution of counts versus PI channel for each pattern class (single, double, triple, quadruple), and the bottom is the expected pattern distribution (smooth lines) plotted over the observed distribution (histogram). If the lower plot shows the model distributions for single and double events diverging significantly from the observed distributions, then the source is piled up.
The source used in this example is too faint to provide reasonable statistics for epatplot and is far from being affected by pile up. In contrast, Figure 7.5 shows an example of a bright source (from a different observation) which is strongly affected by pileup. Note the severe divergence between the model and the observed pattern distribution.
To check for pile up with the GUI:
To check for pile up with the Command Window, type:
The postscript file can be copied to the user's local machine and viewed there.