Analysis and Preliminary Results for the Cosmic Ray Electron Spectrum from CALET

Abstract

CALET (CALorimetric Electron Telescope) mission was launched to the International Space Station (ISS) in August 2015 and has been accumulating scientific data since October 2015. The CALET detector, consisting of a charge detector (CHD), imaging calorimeter (IMC) and total absorption calorimeter (TASC), features a very thick calorimeter of 30 radiation-lengths. Due to its high energy resolution and its ability to discriminate electrons from hadrons, CALET enables a detailed search for various spectral structures in high-energy electron cosmic rays, with the capability to provide the first experimental evidence of the presence of a nearby astrophysical cosmic-ray source. The electron spectrum analysis starts with detailed detector calibrations ranging from detector alignment to energy determination, followed by selection of well-reconstructed singly charged electron tracks, further quality cuts using shower concentration along the shower axis in the bottom layer of IMC, and analysis of the energy deposit distributions in the IMC and TASC layers and the shower shape difference between electromagnetic and hadronic showers. The geometric acceptance and selection efficiencies are calculated using the detailed Monte Carlo simulation of the CALET flight model, in which all of the relevant detector responses are taken into account. Background contamination due to the remaining protons is estimated and subtracted from the surviving events using simulated proton data, which reproduces the current observation results. Various consistency checks between data and MC simulation are performed using flight data. A preliminary spectrum of total electrons (electrons + positrons) was obtained in an energy range of 10 GeV to 1 TeV, based on a limited data sample of 18 months of observation. It will be published soon in a refereed journal.