The attenuation of atmospheric Cerenkov photons

Abstract

Whilst the atmosphere places a limit on the successful applications of many branches of astronomy, it becomes an invaluable tool for the detection of very high energy γ-rays. This thesis is concerned with reducing the systematic uncertainties inherent to using the atmosphere as a detector of very high energy radiation. The interaction processes important to high energy particles are met in the first chapter. The second chapter explores how these interaction processes are responsible for generating observable Cerenkov radiation that can be detected by ground based telescopes. A description of one of these atmospheric Cerenkov telescopes, the University of Durham Mark 6 telescope, is given in chapter 3. A timing analysis was performed on data obtained with this telescope of the high mass X-ray binary Centaurus X-3 and the findings are given in chapter 5. The result of the test for orbital modulation of the VHE γ-ray signal has implications for the possible site of VHE γ-ray emission in this system and for the analysis strategy required to test for modulation of the VHE signal at the pulsar period. One of the findings of the timing analysis was a need for more accurate flux estimates and spectral energy measurements of the VHE γ-ray signal, which requires a greater understanding of the systematic errors inherent to the atmospheric Cerenkov technique. The effective collecting area of a Cerenkov telescope is related to the generation and attenuation of Cerenkov photons in the atmosphere. Uncertainties in the magnitude of the effective area result in errors in the deduced flux, whilst uncertainties in the function of effective area with energy result in errors of the spectral slope determined for any source. By using an inappropriate model for the atmosphere in simulations of atmospheric Cerenkov telescopes a systematic error can be introduced into calculations of the effective area. Chapter 6 compares the effective areas obtained from several model atmosphere types, including a contemporary model of the conditions at the Mark 6 site from data taken with atmospheric monitoring equipment employed in the Mark 6 operations. The findings from this work are then all drawn together in the final chapter, along with a discussion of the future atmospheric monitoring work that will go in to the next generation of atmospheric Cerenkov telescope installations.