The Fermi Large Area Telescope On Orbit: Event Classification, Instrument Response Functions, and Calibration

TL;DR

This paper details the on-orbit performance, calibration, and updated instrument response functions of the Fermi-LAT gamma-ray telescope, including validation methods and systematic error estimation for astrophysical measurements.

Contribution

It provides updated IRFs, calibration procedures, and validation techniques based on flight data, improving data analysis accuracy for the Fermi-LAT mission.

Findings

Updated IRFs account for flight data discrepancies.

Validated IRFs with flight data, reducing uncertainties.

Quantified systematic errors in flux and spectral measurements.

Abstract

The Fermi Large Area Telescope (Fermi-LAT, hereafter LAT), the primary instrument on the Fermi Gamma-ray Space Telescope (Fermi) mission, is an imaging, wide field-of-view, high-energy \gamma-ray telescope, covering the energy range from 20 MeV to more than 300 GeV. During the first years of the mission the LAT team has gained considerable insight into the in-flight performance of the instrument. Accordingly, we have updated the analysis used to reduce LAT data for public release as well as the Instrument Response Functions (IRFs), the description of the instrument performance provided for data analysis. In this paper we describe the effects that motivated these updates. Furthermore, we discuss how we originally derived IRFs from Monte Carlo simulations and later corrected those IRFs for discrepancies observed between flight and simulated data. We also give details of the validations performed using flight data and quantify the residual uncertainties in the IRFs. Finally, we describe techniques the LAT team has developed to propagate those uncertainties into estimates of the systematic errors on common measurements such as fluxes and spectra of astrophysical sources.