Study of astrophysical transients with the MAGIC telescopes

This PhD work is focused on the study of transient sources, astrophysical events exhibiting short-time scale variability. In particular, Gamma-Ray Bursts (GRBs) and Gravitational Waves (GWs) counterparts were searched using the data of the MAGIC telescopes, two Cherenkov telescopes detecting gamma-rays in the energy range 50 GeV-10 TeV (very high energies, VHE) thanks to the Imaging Atmospheric Cherenkov Technique. GRBs are the most violent sources in the Universe, releasing a huge amount of energy in few seconds. They are characterized by a prompt emission, where the bulk of photons in the hard-X and gamma energy range are emitted, and an afterglow phase, which is a long-lasting but dimmer emission observable in some of other bands (optical, infrared, ultraviolet, radio, X, gamma). One of the current questions about GRBs regards their possible High Energy (HE, E<100 GeV) and VHE emissions. Several models were proposed but no clear signature of any of them was found yet. Detecting a GRB in the MAGIC energy range would help in clarifying the emission processes at play. For this purpose, a set of several GRBs observed by the MAGIC telescopes was analyzed searching for a possible gamma-ray signal. None of the GRBs showed any signal, so only upper limits (integral and differential) on the gamma-ray flux could be determined. Nonetheless, these upper limits can be used to test possible emission models in the VHE range. The second part of this work was dedicated to find a possible gamma-ray counterpart to GW candidate events detected by the LIGO/Virgo interferometers. MAGIC performed the follow-up of GW151226, the second detection of a GW signal from a binary black hole system. Since the uncertainty in the position of the event given by LIGO was too large compared to the MAGIC field of view (3.5 degrees), a new analysis method had to be implemented. Usually fluxes and spectra are computed at the nominal position of the source, but in the case of large error boxes the emission could be coming from any point of the field of view. For this reason a new tool built upon the existing MAGIC analysis software was developed in order to compute fluxes and upper limits in a wide region of the observed field of view. The method was used to produce "upper limits skymaps" of the four positions observed after the GW151226 trigger. No gamma-ray signal was found. The last part of the thesis was focused on a technical work on the MAGIC automatic system which receives and processes the transient alerts coming from the Gamma-ray Coordinate Network (GCN), called GSPOT (Gamma-ray Source POinting Target). As an extension to other astrophysical transients, the automatic response to neutrino alerts coming from IceCube was implemented in a similar way to what is currently done with GRB alerts. Preliminary tests performed remotely on GSPOT resulted in a correct processing of the neutrino alerts, with a correct calculation of the observability according to predefined criteria.