Refinement of the High-Energy Gamma-ray Selection for CALET on the International Space Station

Abstract

The Calorimetric Electron Telescope (CALET) is a deep electromagnetic calorimeter designed for the measurement of cosmic-ray electrons on the International Space Station. Deployed on the Exposed Facility of the Japanese Experiment Module since August 2015, it observes cosmic-ray electrons with energies up to above 10 TeV and hadrons up to PeV total energies. It is also sensitive to gamma rays in the energy range from 1 GeV to 1 TeV. At energies above ~100 GeV, sensitivity is lost in the gamma-ray selection as previously defined. This is due in large part to the contamination of the charge measurement in the CHD (Charge Detector) and the IMC (Imaging Calorimeter) by backscattered particles from the electromagnetic shower in the TASC (Total Absorption Calorimeter). In this work, we implement a revision to the photon selection criteria using a significantly increased simulated dataset, increasing CALET's exposure at these energies significantly. Furthermore, the allowable geometry for incident photons is expanded and the energy reconstruction revised to tighten the energy resolution for highly inclined tracks. We validate the distributions used for the selection through comparison with real data, evaluate the changes to the instrument response functions, and assess the implications for the flight data analysis of gamma rays with CALET.