Calorimeter-only analysis of the Fermi Large Area Telescope

TL;DR

This paper introduces the Calorimeter-only event class for the Fermi LAT, aiming to improve high-energy gamma-ray detection by increasing acceptance despite reduced resolution, thus addressing low photon statistics at tens of GeV.

Contribution

The paper presents a novel Calorimeter-only event class that enhances the LAT's acceptance at high energies when tracker data is unavailable.

Findings

CalOnly events increase the effective acceptance of the LAT above tens of GeV.

CalOnly events have poorer angular resolution and background separation compared to standard events.

Preliminary analysis shows potential for improved gamma-ray observations at high energies.

Abstract

Above tens of GeV, gamma-ray observations with the Fermi Large Area Telescope (LAT) can be dominated by statistical uncertainties due to the low flux of sources and the limited acceptance. We are developing a new event class which can improve the acceptance: the "Calorimeter-only (CalOnly)" event class. The LAT has three detectors: the tracker, the calorimeter, and the anti-coincidence detector. While the conventional event classes require information from the tracker, the CalOnly event class is meant to be used when there is no usable tracker information. Although CalOnly events have poor angular resolution and a worse signal/background separation compared to those LAT events with usable tracker information, they can increase the instrument acceptance above few tens of GeV, where the performance of Fermi-LAT is limited by low photon statistics. In these proceedings we explain the concept and report some preliminary characteristics of this novel analysis.