Post-launch performance of the Fermi Large Area Telescope

TL;DR

This paper reviews the initial performance and calibration updates of the Fermi LAT instrument after launch, highlighting improvements in data analysis and response functions for gamma-ray observations.

Contribution

It provides the first comprehensive update on the Fermi LAT's in-flight performance and calibration based on actual flight data, enhancing data analysis accuracy.

Findings

Updated calibrations based on flight data

Improved event reconstruction algorithms

Verification of response functions using celestial sources

Abstract

The Large Area Telescope (LAT) on-board the Fermi Gamma-ray Space Telescope started nominal operations on August 13, 2008, after about 60 days of instrument checkout and commissioning and is currently performing an all-sky gamma-ray survey from 30 MeV to above 300 GeV with unprecedented sensitivity and angular resolution. The LAT pre-launch response was tuned using Monte Carlo simulations and test beam data from a campaign necessarily limited in scope. This suggested a conservative approach in dealing with systematics that affect the reconstruction analysis of the first months of data taking. The first major update of the instrument performance based on flight data is now being completed. Not only are the LAT calibrations now based on flight data, but also the ground event reconstruction has been updated to accommodate on-orbit calibrations, and response was carefully verified using real data from celestial sources. In this contribution we describe the current best knowledge of the instrument, and our plans towards releasing public response functions to support data release in year 2.