In-field array and atmospheric calibration strategy of the Cherenkov Telescope Array Observatory

Abstract

The Cherenkov Telescope Array Observatory (CTAO) will observe very-high-energy gamma rays with unprecedented sensitivity and will study a large variety of gamma-ray sources with high statistics. Precision measurements will allow researchers to search for subtle spectral and morphological features beyond simple power-laws, which may encode information on particle acceleration, source environments, propagation effects, or fundamental physics. To control systematic uncertainties to the necessary level, CTAO has formulated requirements on the knowledge of the absolute energy and flux scales, angular resolution, and gamma-ray direction reconstruction accuracy, among others. CTAO uses the Imaging Atmospheric Cherenkov Telescope technique, in which the atmosphere acts as a calorimeter for gamma-ray-induced atmospheric air showers, and measures the Cherenkov light collected by telescopes exposed to the environment without protective domes. Consequently, both the atmosphere and the telescopes require timely characterization and follow-up calibration. Due to the absence of well-calibrated gamma-ray reference beams, CTAO relies on accurate Monte-Carlo simulations of the atmosphere and detector. Calibration therefore consists of providing accurate time-dependent configurations for these simulations, for each respective time interval of the data taken. All these inputs come along with their own systematic uncertainties, whose combined contributions shall not exceed the global requirement. This paper outlines the leading contributions to the to these uncertainties and presents an in-field array and atmospheric calibration strategy designed to keep the global error budget under strict control. We discuss telescope calibration with local muons, selected gamma-ray samples and dedicated calibrated light sources that illuminates the telescopes with precisely known light intensities. Particularities of the atmosphere above both CTAO sites will be presented and means to monitor both their molecular and aerosol components through a combination of publicly available global data assimilation software, Raman LIDARs and stellar photometry.