What data should be used for source analysis?

Assume that we are analyzing the spectrum of a single source. Because of the large point spread function at low energies (e.g., 68% of the counts will be within 3.5 degrees at 100 MeV), we want to use the counts within a region around the source. Nearby sources will contribute counts to this region, and we want to model them; i.e., to model a single source we are forced to model a handful of sources. Therefore, we need to include counts from an even larger region.

For the greatest accuracy possible in modeling a single source, we should model the entire sky(!), but this is not usually feasible and, in reality, the influence of sources a very great distance away from the source will be greatly attenuated. Thus ,we include sources from a large 'Source Region' and counts from a smaller 'Region of Interest' (ROI).

The positions and spectra of sources in the Source Region outside of the ROI were obtained previously; from a catalog, for example. These sources are included because of their contribution to the counts in the ROI. How we treat the sources in the ROI is under our control: we may wish to fix the parameters of the sources other than the one we are studying at their catalog values, or we might want to perform a fit on the parameters of all of these sources.

To summarize, we will use all of the sources in the Source Region, and determine the size of the Source Region appropriate for our needs from experience and experimentation.

Recommended values. Default values of ROI+10 and ROI+5 degrees are recommended for sources dominated by ~100 MeV and ~1 GeV events, respectively, and all counts in the ROI are included. The appropriate ROI size is determined based on experience and experimentation, but the recommended default values are 20 and 15 degrees, respectively, for sources dominated by ~100 MeV and ~1 GeV events.