Description
The LAT instrument onboard the Fermi satellite is monitoring the universe in high-energy (~GeV) gamma rays. The data is made available to the community only a few hours after the photons have been recorded. FLaapLUC is an automatized pipeline that analyzes the public Fermi-LAT data in search for flaring activity of a list of pre-defined sources.Currently, the necessary computing ressources are provided by the H.E.S.S. collaboration. The use source list comprises all GeV emitters listed in the 4FGL catalog of the Fermi-LAT collaboration.
FLaapLUC comprises a two-step processing of the data: twice a day an aperture photometric analysis is performed around each source using the most recent photon list from Fermi-LAT. This basically assumes that the data set, within 1 degree around the source, is background-free. As soon as the source flux is surpassing a threshold (set to 3sigma above the long-term flux average), a dedicated likelihood analysis of the ROI is started.
In Astro-COLIBRI, these two steps are represented by two different notification types: 'alerts' (i.e. standing for the aperture photometry analysis) and 'analysis results' (i.e. reporting the results from the likelihood analysis). The produced lightcurves, SEDs and a figure showing the energy of the recorded photons as function of time are stored within the Astro-COLIBRI databases and are updated after every new FLaapLUC processing.
The resulting events within Astro-COLIBRI are localized using the coordinates (and their uncertainty) given by the localisation of the 4FGL source that led to the detection of the enhanced gamma-ray emission. Due to the sizeable point-spread-function of the LAT instrument, it happens frequently that multiple 4FGL sources are being associated with the same emission, thus causing multiple alerts with similar lightcurves from the same region of the sky.