Very-High Energy γ-ray observations of the Geminga pulsar with the LST-1 of CTA

The improvements in the ground-based techniques for γ-ray detection, particularly the Imaging Atmospheric Cherenkov Telescopes (IACTs), allowed the development of Very-High Energy (VHE) astronomy. Nowadays, more than 250 sources populate the TeV sky, and the number is expected to rise when the Cherenkov Telescope Array Observatory (CTAO) will see its first light. It will consist of two arrays composed of IACTs with three different designs to cover the energy range from 20 GeV to 300 TeV. The telescope design tailored for the lowest energies is the Large-Sized Telescope (LST) and the first one, dubbed LST-1, has been operating since 2019, focusing on extragalactic sources, transients and pulsars. The detection of VHE emission from pulsars was one of the most unexpected discoveries of the last few years. Up to now, only three VHE pulsars are known, and the mechanism responsible for their emission is still unclear. The last discovered one is Geminga, which was detected in 2020 by the MAGIC Collaboration. Geminga was observed also by LST-1 and, for this thesis, 21 hours of good-quality data with a zenith cut at 25° were analysed. The second peak of the light curve was detected at an 8σ significance, and the result of the power law fit of its data was compatible with the one of MAGIC. A joint fit with Fermi-LAT and MAGIC showed the limitations of the present theories, based on curvature radiation, in explaining the VHE emission, and the need for more studies to investigate the presence of additional mechanisms, such as Inverse Compton. Moreover, what has been found proves that the LST-1 has excellent low-energy performance: with one-fourth of MAGIC’s observation time and only one telescope, a stronger signal detection was obtained. This is a crucial result for the science verification of the instrument since it shows that LSTs can consistently improve the performance of current-generation instruments and increase the number of known VHE pulsars.