The astrophysical analysis of LAT data begins with a list of counts detected by the LAT. As described above, this list results from processing by the LAT instrument team that reconstructs events from the signals from different parts of the LAT, and categorizes these events as photon or non-photon. Note that we are calling events categorized as photons 'counts.'
While qualitative exploration of the data can suggest the presence of sources (i.e., by the spatial clustering of photons) and time variability, quantitative analysis requires fitting models to the data. This approach is necessitated by the LAT instrument's energy-dependent PSF and geometry-dependent effective area, by the nature of the gamma-ray sky backgrounds, and by the way the Fermi observatory is operated (scanning the sky in survey mode ~90% of the time).
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