Estimation of the height of the first interaction in gamma-ray showers observed by Cherenkov telescopes

Abstract

Very high energy gamma rays entering the atmosphere initiate Extensive Air Showers (EAS). The Cherenkov light induced by an EAS can be observed by ground-based telescopes to study the primary gamma rays. An important parameter of an EAS, determining its evolution, is the height of the first interaction of the primary particle. However, this variable cannot be directly measured by Cherenkov telescopes. We study two simple, independent methods for the estimation of the first interaction height. We test the methods using the Monte Carlo simulations for the 4 Large Size Telescopes (LST) that are part of the currently constructed Cherenkov Telescope Array (CTA) Observatory. We find that using such an estimated parameter in the gamma/hadron separation can bring a mild improvement (∼ 10 − 20%) in the sensitivity in the energy range ∼ 30 − 200 GeV.

V-shaped Cherenkov images of magnetically-separated gamma-rays

“…The extraction of signal amplitudes from simulated waveforms, image cleaning and its parametrization, γ/hadron separation and classical stereo reconstruction and energy estimation is done using MARS/Chimp chain [25,2,26]. The generated MC simulations are described in more detail in [27].…”

Usage of the estimation of the height of the first interaction

“…Therefore, we use the angular distance between the closest pixel and the reconstructed source position to calculate the height of the first interaction. Using the reconstructed impact position and applying a simple correction for the pixalization of the camera (see [9] for detailed description) this angular distance is converted into the estimation of the height at which the first light from the shower has been observed.…”

“…Air showers were simulated with CORSIKA [11], version 6.990, while the telescope response was obtained using sim_telarray [12]. The detailed information about the simulations used as well as about the analysis procedure are given in [9] and references therein.…”

“…In [9] we studied two simple methods that can be used for the estimation of the first interaction height which can be applied to arrays of Cherenkov telescopes. One of the methods, based on the angular distance between the estimated source position, and the closest pixels that survived the image cleaning, has allowed for a mild gain in the sensitivy in the low-energy range for a subarray of LSTs.…”

Extracting Gamma-Ray Information from Images with Convolutional Neural Network Methods on Simulated Cherenkov Telescope Array Data

“…This quantity can be difficult to reconstruct if the number of triggered telescopes is small. Moreover, the algorithm [37] used to reconstruct this variable needs knowledge about the physics interaction and detector response. In contrast CNN algorithm needs no additional physical knowledge except what is in the simulation and we use the same CNN architecture as for directional reconstruction.…”