The throughput calibration of the VERITAS telescopes

C Adams ,G Maier,Q Feng,M Santander,J H Buckley,B Hona,S O’Brien,R R Prado,T J Williamson,A Brill,D A Williams,A Furniss,A Weinstein,A Falcone,T K Kleiner,J Quinn,D Hanna,R Mukherjee,M Lundy,M Pohl,K Pfrang,N Park,J L Christiansen,W Benbow,E Pueschel,P Moriarty,S Kumar,K Ragan,M J Lang,T B Humensky,W Jin,D Ribeiro,J Holder,O Hervet,E Roache,P T Reynolds,C Giuri,J P Finley,M Nievas Rosillo,L Fortson,J Ryan ,Gregory Foote ,Tarek Hassan ,Sameer Patel ,Philip Kaaret

doi:10.1051/0004-6361/202142275

Abstract

Context. The response of imaging atmospheric Cherenkov telescopes to incident γ-ray-initiated showers in the atmosphere changes as the telescopes age due to exposure to light and weather. These aging processes affect the reconstructed energies of the events and γ-ray fluxes. Aims. This work discusses the implementation of signal calibration methods for the Very Energetic Radiation Imaging Telescope Array System (VERITAS) to account for changes in the optical throughput and detector performance over time. Methods. The total throughput of a Cherenkov telescope is the product of camera-dependent factors, such as the photomultiplier tube gains and their quantum efficiencies, and the mirror reflectivity and Winston cone response to incoming radiation. This document summarizes different methods to determine how the camera gains and mirror reflectivity have evolved over time and how we can calibrate this changing throughput in reconstruction pipelines for imaging atmospheric Cherenkov telescopes. The implementation is validated against seven years of observations with the VERITAS telescopes of the Crab Nebula, which is a reference object in very-high-energy astronomy. Results. Regular optical throughput monitoring and the corresponding signal calibrations are found to be critical for the reconstruction of extensive air shower images. The proposed implementation is applied as a correction to the signals of the photomultiplier tubes in the telescope simulation to produce fine-tuned instrument response functions. This method is shown to be effective for calibrating the acquired γ-ray data and for recovering the correct energy of the events and photon fluxes. At the same time, it keeps the computational effort of generating Monte Carlo simulations for instrument response functions affordably low.

Highlights

When energetic γ-rays or charged particles enter the atmosphere, they generate a cascade of secondary particles through pair production, bremsstrahlung emission, and, for the case of hadronic showers, fragmentation and decay of unstable mesons (π0, π±, and K±)
We show the results for boosted decision trees (BDTs)-based moderate cuts with a minimum telescope multiplicity of two, which provides a balance between optimizing sensitivity and maintaining a low energy threshold
The described methodology, well defined, will continue to be used for the upcoming years of Very Energetic Radiation Imaging Telescope Array System (VERITAS) operations and we are confident it will serve for other experiments as well

Summary

Introduction

When energetic γ-rays or charged particles (typically protons, atomic nuclei, or electrons) enter the atmosphere, they generate a cascade of secondary particles ( known as an extensive air shower or EAS) through pair production, bremsstrahlung emission, and, for the case of hadronic showers, fragmentation and decay of unstable mesons (π0, π±, and K±). The resulting ultrarelativistic particles, traveling faster than the local speed of light in the atmosphere, produce coherent polarization of the dielectric medium. This in turn produces beamed Cherenkov radiation in the forward direction, forming a light pool at ground level of approximately 150 m radius. The number of Cherenkov photons emitted in the shower is approximately proportional to the energy of the primary particle (Hillas 1985; de Naurois & Mazin 2015). This is true for air showers that are generated by primary electrons and γ-rays

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Conclusion