Sharper Fermi LAT Images: Instrument Response Functions for an Improved Event Selection

Abstract

The Large Area Telescope (LAT) on the Fermi Gamma-ray Space Telescope has a point-spread function (PSF) with large tails, consisting of events affected by tracker inefficiencies, inactive volumes, and hard scattering; these tails can make source confusion a limiting factor. The parameter CTBCORE, available in the publicly available Extended Fermi LAT data (available at http://fermi.gsfc.nasa.gov/ssc/data/access/), estimates the quality of each event's direction reconstruction; by implementing a cut in this parameter, the tails of the PSF can be suppressed at the cost of losing effective area. We implement cuts on CTBCORE and present updated instrument response functions derived from the Fermi LAT data itself, along with all-sky maps generated with these cuts. Having shown the effectiveness of these cuts, especially at low energies, we encourage their use in analyses where angular resolution is more important than Poisson noise.