Multi-wavelength afterglow numerical code and data analysis of MAGIC very high energy emission from gamma-ray bursts

This PhD thesis is focused on the study of very high energy (E > 100 GeV, VHE) emission from a class of extragalactic transient sources called Gamma-Ray Bursts (GRBs). In particular, I have i) performed data analysis of GRB observations by the MAGIC telescopes; ii) developed a numerical code to exploit such observations and investigate the properties of such astrophysical sources. Despite been discovered more than 50 years ago, GRBs are still a hot topic in Astrophysics. They are extremely energetic phenomena characterized by two emission phases. The first one, called prompt phase, is a rapid and irregular emission in the X and gamma energy range which lasts from milliseconds to thousands of seconds. The second one, called afterglow emission, consists in a long-lasting (days to months) emission covering the entire electromagnetic spectrum, from radio up to gamma-rays and interpreted as the synchrotron radiation produced in the external forward shock scenario. The possible extension of the GRB emission in the high energy (0.5 MeV < E < 100 GeV, HE) and to the very high energy domain has always been one of the most debated open questions in GRB physics. The HE observations have revealed an emission component delayed and lasting longer with respect to the prompt one. Moreover, the highest energy photons observed by Fermi-LAT gave the first evidence that a different radiation mechanism with respect to the synchrotron radiation may be needed to explain such energetic emission and point towards the possibility that a VHE component in GRBs is present. A firm conclusion could not be reached until 2019, when an unprecedented discovery was performed: the ground-based imaging Cherenkov telescopes MAGIC and H.E.S.S. revealed for the first time ever the presence of a VHE emission component in GRB afterglows up to TeV energies. Such detections gave birth to unique studies concerning the radiation processes involved in GRBs. For one of these events, namely GRB190114C, the VHE emission was interpreted as produced via the Synchrotron Self Compton (SSC) mechanism. First, in this thesis I will present MUSE-GRB, a numerical code which I developed to simulate the multi-wavelength GRB afterglow spectra and light curves in the external forward shock scenario. In this code the dynamical evolution of a blastwave interacting with an external medium is reproduced and the time-evolving kinetic equations for electrons, protons and photons are solved. As a result, the afterglow spectra and the light curves at different times and frequencies can be calculated. This flexible and very complete code can be applied to GRB observational data to model the multi-wavelength afterglow emission, thus inferring constraints on several properties of the GRB physics. The code has been intensively tested with analytical prescriptions and with a similar numerical code available in literature. Such tool is able to model the observational data coming from the MAGIC and the H.E.S.S. telescopes and verify their consistency with a SSC origin. In the second part of this thesis I will present the MAGIC data analysis of two GRBs detected in the VHE band, namely GRB190114C and GRB190829A. I performed the dedicated MAGIC data analysis of GRB190114C in the context of a multi-wavelength study of the emission. The MAGIC data were analyzed in several time intervals to investigate a possible spectral evolution in the VHE band. The statistical error and several sources of systematic error were assessed in order to evaluate the firmness and the stability of the data analysis. GRB190829A, successfully detected in the VHE band by the H.E.S.S. telescopes, was also observed by the MAGIC telescopes. Preliminary MAGIC data analysis shows a possible hint of detection. In the context of the study and interpretation of the VHE emission coming from GRBs some applications of MUSE-GRB to GRB190114C and GRB190829A are also presented.