Atmospheric monitoring and gamma-ray data analysis with the H.E.S.S. telescope array

Abstract

The High Energy Stereoscopic System (H.E.S.S.) consists of an array of four ground-based imaging atmospheric Cherenkov telescopes. The aim of the system is to detect Cherenkov radiation emitted from extensive air showers caused by very high energy gamma-ray radiation interacting with the atmosphere. The implementation of more than one telescope allows multiple views of a single extensive air shower, providing a more accurate reconstruction of the event and offering greater cosmic ray background suppression, allowing better analysis of gamma rays. Unlike charged particles, gamma rays reach the Earth in the direction in which they were emitted thus allowing us to investigate the emission mechanisms and properties of various gamma-ray sources. Since Cherenkov radiation is produced and propagates through the atmosphere, the atmosphere plays a large role in the detection of gamma rays and must be monitored carefully. This thesis will explore the effects of the atmosphere on the H.E.S.S. telescopes and gamma-ray analysis and is structured as follows: The first chapter highlights all the important aspects of very high energy (VHE) gamma-ray astronomy focusing on the H.E.S.S. system. Chapter 2 gives a general overview of active galactic nuclei (AGN) and the extragalactic background light (EBL) as a precursor to data analysis in Chapter 4. Chapter 3 discusses the atmosphere and atmospheric monitoring performed at the H.E.S.S. site. I present data analysis of the four atmospheric monitoring instruments revealing seasonal trends and cross-correlations between instruments. Chapter 4 contains the heart of the thesis, where all aspects from the previous chapters are integrated to show the effects of the atmosphere on the H.E.ร.ร. system and subsequent AGN and EBL data analysis. I first compare trigger rate data from the telescopes and data from the atmospheric monitoring instruments. It is shown that backscatter, sky radiance, and transmissivity are correlated to the trigger rate to various degrees. It is then shown that gamma-ray data is less affected by varying atmospheric conditions, especially for the higher energy gamma rays. Active atmospheric calibration is then presented as a way to correct gamma-ray data when there is a large amount of low-lying aerosols in the atmosphere. This method is subsequently used to correct the gamma-ray fluxes for 2 AGN, PKS 2155-304 and H 2356-309. It is shown that the correction works best for the higher flux source PKS 2155-304 rather than H 2356-309 whose flux is often consistent with zero. These AGN are then used to calculate upper limits to the EBL using a simple monoenergetic model. The upper limits to the EBL in the wavelength range of 0.4-1 μm are shown to be consistent with the results from the most exhaustive study of the EBL testing over 8 million EBL shapes. Chapter 5 is the concluding chapter discussing future work to be done at H.E.S.S. site, with the addition of a new LIDAR and VHE telescope, and beyond.