Quality and Stability of Cosmic Ray Energy Assignments at the Pierre Auger Observatory

Abstract

One of the biggest mysteries in astrophysics is the origin and nature of ultrahigh energy cosmic rays (UHECRs). Measuring the energies accurately is vital in understanding sources of cosmic rays and the physical mechanisms behind their acceleration. The energy spectrum, which describes flux of cosmic rays as a function of energy, can give insight to their possible sources. Many observatories around the world have been constructed to detect UHECRs and measure their energies to the highest detection accuracy possible. Due to the low flux of cosmic rays at the highest energies, various detectors are deployed spanning extremely large areas to detect extensive air showers initiated by cosmic rays arriving at the top of Earth’s atmosphere. The signal strengths, timing information and air shower profiles can then be measured to accurately obtain energies of cosmic rays. One of the most notable detectors of UHECRs is the Pierre Auger Observatory. It is the largest cosmic ray detector in the world, located in western Argentina. It adopts a hybrid detection technique through the use of two independent detectors known as the fluorescence detector (FD) and the surface detector (SD). The hybrid design aids in eliminating model dependencies and provides important cross-checks for accurate air shower reconstruction. This thesis investigates the stability (with time and position) of FD and SD energy assignments at the Pierre Auger Observatory. Chapters 1, 2 and 3 describe cosmic rays, detection methods and the Pierre Auger Observatory. The SD, FD and hybrid design are outlined followed by details of energy reconstruction. Analysis begins in Chapter 4 with the FD energy over SD signal ratio. The FD stability is probed through a quantitative analysis of this ratio in a 14-year long dataset subject to varying conditions. Trends in the ratio seen by each telescope in the FD are also analysed for correlations with cleaning campaigns done to the UV filters and mirrors. Chapter 5 presents an analysis on SD energy stability, demonstrating evolution of SD event rates above a full efficiency threshold energy with time and station position across the array. Results are compared with previous work and possible causes for the observed drift are suggested. Chapter 6 details an analysis on SD event rate evolution with time and distance from each FD site. This is done through the selection of stations involved with hybrid events. Hybrid event rates are then assigned using yearly SD rates. Observed trends are compared with structures in the SD rates and energy ratio. It was found that the time dependence in the energy ratio and rates are statistically significant. On average, energies measured by the SD are increasing with time and this contributes to the drift in the energy ratio. Position dependence also exists across the array and this was attributed to a decay in sensitivity of some stations in the SD. Other causes for the dependencies are suggested, but further work is needed to investigate the extent of their contributions. Chapter 7 summarises the main results and outlines some further investigations.