Sharper Fermi LAT Images: instrument response functions for an improved event selection

TL;DR

This paper improves Fermi LAT gamma-ray images by implementing cuts on the CTBCORE parameter, resulting in sharper point spread functions and better source localization, especially at low energies.

Contribution

The authors develop and provide updated instrument response functions using CTBCORE cuts to enhance Fermi LAT image resolution.

Findings

Reduced point spread function tails with CTBCORE cuts

Improved angular resolution at low energies

Updated all-sky maps with enhanced image sharpness

Abstract

The Large Area Telescope on the Fermi Gamma-ray Space Telescope has a point spread function with large tails, consisting of events affected by tracker ineffiencies, 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, estimates the quality of each event's direction reconstruction; by implementing a cut in this parameter, the tails of the point spread function 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.