The On-orbit Calibrations for the Fermi Large Area Telescope

TL;DR

This paper details the calibration procedures and results for the Fermi Large Area Telescope on orbit, ensuring accurate measurements of gamma-ray sources through various calibration techniques and demonstrating their stability over time.

Contribution

It presents the first comprehensive on-orbit calibration results for the Fermi LAT, including methods and stability analysis, which are essential for accurate astrophysical measurements.

Findings

Calibration parameters remained stable over time.

Minor adjustments were made since launch.

Calibration techniques effectively used astrophysical sources and cosmic rays.

Abstract

The Large Area Telescope (LAT) on--board the Fermi Gamma ray Space Telescope began its on--orbit operations on June 23, 2008. Calibrations, defined in a generic sense, correspond to synchronization of trigger signals, optimization of delays for latching data, determination of detector thresholds, gains and responses, evaluation of the perimeter of the South Atlantic Anomaly (SAA), measurements of live time, of absolute time, and internal and spacecraft boresight alignments. Here we describe on orbit calibration results obtained using known astrophysical sources, galactic cosmic rays, and charge injection into the front-end electronics of each detector. Instrument response functions will be described in a separate publication. This paper demonstrates the stability of calibrations and describes minor changes observed since launch. These results have been used to calibrate the LAT datasets to be publicly released in August 2009.