A Likelihood Method for Determining the On-orbit Point-Spread Function of the Fermi Large-Area Telescope

Abstract

The Large-Area Telescope (LAT) on the <italic>Fermi</italic> Gamma-Ray Space Telescope is a pair-conversion gamma-ray telescope with unprecedented capability to image astrophysical gamma-ray sources between 20 MeV and 300 GeV. The pre-launch performance of the LAT, decomposed into effective area, energy and angular dispersions, were determined through extensive Monte Carlo (MC) simulations and beam tests. The point-spread function (PSF) characterizes the angular distribution of reconstructed photons as a function of energy and geometry in the detector. Here we present a set of likelihood analyses of LAT data based on the spatial and spectral properties of sources, including a determination of the PSF on orbit. We find that the PSF on orbit is generally broader than the MC at energies above 3 GeV and consider several systematic effects to explain this difference. We also investigated several possible spatial models for pair-halo emission around BL Lac AGN and found no evidence for a component with spatial extension larger than the PSF.