Fermi Gamma-ray Space Telescope

Time-Series Analysis with Likelihood

Although the LAT instrument response functions capture the long term behavior of the LAT extremely well, there are several effects which have been averaged over, but which can vary significantly (on the 10% level or so) on short timescales such as hours, days and weeks depending on the location of the source under study. In particular:

  • The phi dependence of the effective area. This can be taken into account by including the phi dependence when making the livetime cubes with gtltcube.
  • The livetime dependence of the effective area. This can be studied by splitting the data into subsets based on the livetime fraction with gtmktime.
  • The theta dependence of the effective area, which is related to the ratio of front converting to back converting events. This can be studied by breaking the data into time ranges when the source is in different theta ranges.
  • Particle background contamination rates, which will be picked up by the isotropic diffuse template. This can be studied by looking at the variation of the diffuse templates, and by comparing P7REP_SOURCE_V15 to P7REP_ULTRACLEAN_V15 selections.
  • The theta dependence of the PSF, which is only present in P7REP_TRANSIENT_V15 events. This can be studied by comparing results obtained with P7REP_TRANSIENT_V15 to those with the P7REP_SOURCE_V15 IRF.

In general, when doing variability analyses, the LAT team recommends using nearby bright source as control samples, as they are very unlikely to exhibit the same variability as the source under study, but they will be affected by the same systematics.

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