Fermi Gamma-ray Space Telescope

Introducing the Pass 8 Event Types: Event-by-Event Reconstruction Quality in Fermi-LAT Data

M. Testa
R. Caputo, M. Wood, On Behalf of the Fermi LAT Collaboration

Abstract:

The new Fermi-LAT event-level analysis, Pass 8, includes event type partitions based on a specific characteristic (e.g., the quality of the direction or energy reconstruction), beyond the previous front and back partitions of Pass 7. Using an event-by-event measure of the quality of the reconstruction direction, photon candidates within an event class are divided into quartiles (the PSF event types). The lowest quartile (PSF 0) has a 68% containment angle of ~.5 at 10GeV, while the best quality quartile (PSF3) has a 68% containment angle of ~.1 at 10GeV. Similarly, the EDISP event-type partitioning uses an event-by-event measure of the quality of the reconstructed energy. The lowest quality quartile (EDISP0) has a 68% containment energy resolution of 15% at 10 GeV, while the best quality quartile (EDISP3) has a 68% containment energy resolution of 4% at 10 GeV. This allows for analysis methods that take advantage of potentially useful extra information that the reconstruction makes available on an event-by-event basis, information that is effectively lost when averaging over the entire data set. Both the Spectral Line and Dwarf Spheroidal papers used these new event types to increase sensitivity in their likelihood analyses.