MAGIC observations with bright Moon and their application to measuring the VHE gamma-ray spectral cut-off of the PeVatron candidate Cassiopeia A
School
Universitat Autònoma de Barcelona - Institut de Física d'Altes Energies
Edifici Cn, Universitat Autònoma de Barcelona (UAB), E-08193 Bellaterra (Barcelona), Spain
External url: https://www.tdx.cat/handle/10803/664122
Abstract:
Cosmic rays are atomic nuclei that are constantly bombarding the Earth. It is widely accepted that these nuclei with energies up to ~3 PeV are of Galactic origin. But the question about where in the Galaxy and how they are accelerated still remains unanswered. For several years the most popular hypothesis has been that they are accelerated in supernova remnants. In this thesis I present the details of a deep observation campaign on the young supernova remnant Cassiopeia A, one of the most promising candidates to be a PeVatron, a system capable of accelerating cosmic rays up to PeV energies. The observations were performed with the MAGIC telescopes, that observe very high energy (VHE, E > 50 GeV) gamma rays, between December 2014 and October 2016, acquiring 191 hours of good-quality data. Accumulating a large amount of observation hours was indispensable to obtain a precise measurement of the spectrum and it was possible thanks to the possibility of extending the MAGIC duty cycle by operating the telescopes under bright moonlight. I worked in the optimization of moonlight observations with MAGIC, both during the operation of the telescopes and at the data analysis stage. I discuss the details of these developments and the resulting performance. With more than 70% of the data obtained under moonlight, I was able to obtain the most precise spectrum of Cassiopeia A to date at VHE. The obtained spectrum shows for the rst time 4:9 evidence of a cut-o at E = 3,5 (+1,6\—1,0) stat (+0,8\−0,9) sys TeV. The modelling of the spectrum suggests that the bulk of the gamma-rays emitted can be attributed to a population of high-energy protons with spectral index ~2.2 and energy cuto at ~10 TeV. This implies that, assuming there is no signicant cosmic-ray diusion, Cassiopeia A cannot be a PeVatron at its present age.
Keywords:
Cosmic rays, PeVatron, Supernova remnant, Moonlight observations, IACT, VHE gamma rays, MAGIC, Light-Trap, SiPM