Gamma-Ray Transients observed with the Fermi Large Area Telescope

Many observed phenomena in the universe are not static: this is why time- domain astrophysics is a key field of current astronomy and astrophysics. The temporal domain offers an important window on the understanding of extreme phases of stellar and galaxy evolution through studies of novae, supernovae, Gamma-Ray Bursts (GRBs), pulsars, Active Galactic Nuclei (AGN) to list only a few. Variable objects show some different characteristic variability and a lot of information on the physical processes at work comes from measuring luminosity and spectral variations over time. Launched in June 2008, during its first eight years of operation the Fermi Gamma-Ray Space Telescope has confirmed that the gamma-ray sky is highly dynamic on all time scales, from μs to years, providing insight into extreme physical conditions. My research activity is focused on the analysis of transient gamma-ray sources observed by the Large Area Telescope (LAT), the main instrument on-board Fermi. The variable sources analyzed in this thesis spans different observing time scales: - GRBs and solar flares show an impulsive and short phase that can last from ∼ ms to some hundreds of seconds, and a time-extended phase observed at higher energies that can lasts several hours;
- AGN show variability from hours to days;
- novae, whose high-energy transient emission lasts for weeks. In chapters number 2 and 3, I will present the spectral analysis of the impulsive phase of gamma-ray bursts and solar flares using data from the LAT and the GBM. The goal of of this work has been to develop a semi-automatic analysis-pipeline to optimize the source selection and the modelling of background emission in order to better constrain spectral features and infer important physical information on emission processes at work. In chapter 4 and 5, I will show the analysis performed on LAT data for AGN and novae, in the context of coordinated very-high energy (∼ TeV) observations with the Major Atmospheric Gamma Imaging Cherenkov (MAGIC) telescopes. The aim of Chapter 1 is to present the main features of the observatories whose data have been used for my analyses.