XMM Examples - Source and Background counts

Counting the source and background events is one of the main tasks to check the variability of a source. However, there is no task in the XMMSAS that can be simply used on the eventfile directly. In principle there are two ways to get the counts or count rates for sources observed by XMM: a) create eventfiles for the counts selected in the source and background regions, or b) create spectra or light curves for the source and background regions.
Create a filtered event file, spectrum or a light curve as explained in EPIC PN data pages or the EPIC MOS data pages for source and background regions.

The easiest is to count the counts from an eventfile. It is just the number of rows in the event file. This gives you the exact number of counts, except if you have a bright source which shows pile-ups. In this case the number of counts you measure is lower than the real number from the source. Unfortunately there is no way to reconstract the number from pile-uped sources.

If you have a spectrum or lightcurve file, use the FTOOLS fstatistic or fv with the statistics function on the column COUNTS to get the mean of the column COUNTS. multiply this by the number of rows in the column. This gives you the number of counts as well.

To get the background subtracted perform the same steps as explain above to the background region.

Let and the number of photon counts for source+background and the background, and and the sizes of the source+background and background regions. The the background subtracted counts are calculated by:

with the corresponding statistical error:
As an example: You measure 5000 source+background photons in a circular region with a radius of 5 arcminutes and 1000 background photons in a circle of a radius of 10 arcminutes. The the background corrected source counts are:
       4750 = 5000 - 25/100 * 1000
   
and the statistical error of:
       71.2 = (5000 + 625/10000 * 1000)**0.5
   