Determination of the Point-Spread Function for the Fermi Large Area Telescope from On-orbit Data and Limits on Pair Halos of Active Galactic Nuclei

Understand

The Large Area Telescope (LAT) on the Fermi Gamma-ray Space Telescope is a pair-conversion telescope designed to detect photons with energies from 20 MeV to > 300 GeV.

• The pre-launch response functions of the LAT were determined through extensive Monte Carlo simulations and beam tests.
• The point-spread function (PSF) characterizing the angular distribution of reconstructed photons as a function of energy and geometry in the detector is determined here from two years of on-orbit data by examining the distributions of \gamma rays from pulsars and active galactic nuclei (AGN).
• Above 3 GeV, the PSF is found to be broader than the pre-launch PSF.