Thesis defense of Rune Michael Dominik

The Cherenkov Telescope Array Observatory (CTAO) will be the next-generation ground-based very-high-energy (VHE) gamma-ray observatory once its construction and commissioning are finished. Like its predecessors, CTAO relies on Instrument Response Functions (IRFs) to relate the observed and reconstructed properties to the true  ones of the primary gamma-ray photons and thus reconstruct spectral and spatial information of the observed sources. As IRFs are derived from Monte Carlo simulations and depend on observation conditions like telescope pointing and atmospheric transparency, producing a complete set of IRFs is a time-consuming task and not feasible when analyzing data on short timescales. To facilitate the production of optimized IRFs in such scenarios, this work studies the use of inter- and extrapolation algorithms to quickly compute IRFs from a pre-computed grid for the Large-Sized Telescope prototype (LST-1) using the pyirf python software package. As some constituents of an IRF are given as probability distributions, specialized methods are needed.
Using 35.9 h of LST-1 Crab Nebula observation taken with zenith angles up to 35 deg, this thesis shows the compatibility of estimated IRFs and a nearest neighbor approach on the provided LST-1 simulation grid. When using sparser grids, estimated IRFs maintain a stable performance well beyond the point where the nearest neighbor approach can no longer yield reasonable results. Applying estimated IRFs to observations of NGC 1275 from December 2022 and January 2023 in the same zenith range shows clear signs of two flares in this period, matching the signature obtained from past events. Estimated IRFs present themselves to be fully capable of being used with LST-1 analyses in a zenith range of up to 35 deg.

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