Performance and systematic uncertainties of CTA-North in conditions of reduced atmospheric transmission

TL;DR

This study assesses how clouds with varying transmission and altitude impact the performance and systematic uncertainties of the CTA-North gamma-ray observatory through detailed simulations, highlighting the importance of calibration.

Contribution

The paper introduces a semi-analytical model to quantify and understand the effects of cloud transmission and altitude on CTA-North's performance degradation.

Findings

Energy resolution worsens by up to 30% below 1 TeV with low cloud transmission.

Sensitivity can decrease by up to 60% at 40 GeV due to clouds.

Angular resolution varies up to 10% depending on cloud properties.

Abstract

The Cherenkov Telescope Array (CTA) is the next-generation stereoscopic system of Imaging Atmospheric Cherenkov Telescopes (IACTs). In IACTs, the atmosphere is used as a calorimeter to measure the energy of extensive air showers induced by cosmic gamma rays, which brings along a series of constraints on the precision to which energy can be reconstructed. The presence of clouds during observations can severely affect Cherenkov light yield, contributing to the systematic uncertainty in energy scale calibration. To minimize these systematic uncertainties, a calibration of telescopes is of great importance. For this purpose, the influence of cloud transmission and altitude on CTA-N performance degradation was investigated using detailed Monte Carlo simulations for the case where no action is taken to correct for the effects of clouds. Variations of instrument response functions in the presence of clouds are presented. In the presence of clouds with low transmission ($\leq$ 80%) the energy resolution is aggravated by 30% at energies below 1 TeV, and by 10% at higher energies. For higher transmissions, the energy resolution is worse by less than 10% in the whole energy range. The angular resolution varies up to 10% depending both on the transmission and altitude of the cloud. The sensitivity of the array is most severely reduced at lower energies, even by 60% at 40 GeV, depending on the clouds' properties. A simple semi-analytical model of sensitivity degradation has been introduced to summarize the influence of clouds on sensitivity and provide useful scaling relations.