Influence of aerosols from biomass burning on the spectral analysis of Cherenkov telescopes

TL;DR

This paper investigates how aerosols from biomass burning affect atmospheric transparency and the spectral analysis of Cherenkov telescopes, introducing a new monitoring metric to improve calibration accuracy.

Contribution

It introduces the Cherenkov transparency coefficient, a new atmospheric monitoring metric that isolates atmospheric effects from hardware changes in Cherenkov telescope observations.

Findings

The Cherenkov transparency coefficient correlates positively with satellite-based atmospheric optical depth measurements.

Long-term aerosol variations from biomass burning impact the calibration and spectral results of Cherenkov telescopes.

The new metric helps identify observation periods affected by atmospheric aerosols.

Abstract

During the last decade, imaging atmospheric Cherenkov telescopes (IACTs) have proven themselves as astronomical detectors in the very-high-energy (VHE; E>0.1 TeV) regime. The IACT technique observes the VHE photons indirectly, using the Earth's atmosphere as a calorimeter. Much of the calibration of Cherenkov telescope experiments is done using Monte Carlo simulations of the air shower development, Cherenkov radiation and detector, assuming certain models for the atmospheric conditions. Any deviation of the real conditions during observations from the assumed atmospheric model will result in a wrong reconstruction of the primary gamma-ray energy and the resulting source spectra. During eight years of observations, the High Energy Stereoscopic System (H.E.S.S.) has experienced periodic natural as well as anthropogenic variations of the atmospheric transparency due to aerosols created by biomass burning. In order to identify data that have been taken under such long-term reductions in atmospheric transparency, a new monitoring quantity, the Cherenkov transparency coefficient, has been developed and will be presented here. This quantity is independent of hardware changes in the detector and, therefore, isolates atmospheric factors that can impact the performance of the instrument, and in particular the spectral results. Its positive correlation with independent measurements of the atmospheric optical depth (AOD) retrieved from data of the Multi-angle Imaging SpectroRadiometer (MISR) on board of the Terra NASA's satellite is also presented here.