Check for Pile Up

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:

1)

In the Remote Directory List, highlight the name of the filtered event list that was made when the spectrum was extracted, mos1_filt_time_source.fits. In the the Available Tools window, call epatplot.

2)

In the epatplot window, confirm that the event file is in the ``Name of input events file'' text box. Next to ``Whether to determine background subtracted pattern fractions'', click ``yes'', and in the text box just above, enter the name of the background spectrum event list (mos1_filt_time_bkg.fits). Enter the name of the output file; for this example, we will use mos1_pat.ps. Directly below, click ``yes'' next to ``Use plotfile name from parameter''.

3)

Click ``Run''.

To check for pile up with the Command Window, type:

epatplot set=mos1_filt_time_source.fits plotfile=mos1_pat.ps
$$ useplotfile=yes withbackgroundset=yes backgroundset=mos1_filt_time_bkg.fits

The postscript file can be copied to the user's local machine and viewed there.

Figure 7.4: The output of epatplot for a very faint source without pileup. Note that in the lower plot, for energies less than $\sim$ 1500 eV, there are too few X-rays for epatplot to model.

Figure 7.5: The output of epatplot for a heavily piled source. In the lower plot, there are large differences between the predicted and observed pattern distribution at energies above $\sim$ 1000 eV.