Abstract: Time-domain astronomy enables the study of transient sources whose emissions vary from fractions of a second to years. These events produce phenomena across the electromagnetic spectrum and may emit gravitational waves or neutrinos. Transient, energetic gamma-ray emission (above 100 MeV) is connected to drastic changes in the surrounding environment of an astrophysical object or even its partial or total destruction. Despite recent advances, the mechanisms driving the production of gamma rays in transient sources remain poorly understood.
Gamma rays from tens of GeV to hundreds of TeV can be detected with imaging atmospheric Cherenkov telescopes (IACTs). This Thesis focuses on transient sources studied with the next generation of IACTs that will form the Cherenkov Telescope Array Observatory (CTAO), particularly its Large-Sized Telescopes (LSTs), optimised for low-energy gamma rays. The first LST (LST-1) is already operational, with three more LSTs under construction.
We conducted a detailed study of novae, supernovae, and gamma-ray bursts (GRBs). For RS Oph, the first nova detected at very-high-energy (VHE; 100 GeV<E<100 TeV) gamma rays, we modelled its gamma-ray emission using data from LST-1, Fermi-LAT, MAGIC, and H.E.S.S., showing that its gamma-ray emission can be explained in a hadronic scenario. We also assessed the prospects for future nova detections with CTAO. Observations of two nearby supernovae, 2023ixf and SN 2024bch, yielded no detections but allowed constraints on their VHE gamma-ray emission. For GRB 221009A, the brightest known GRB, LST-1 data revealed a 4.1σ significance excess, helping disentangle the degeneracy among models that are consistent with the broad-band emission of this event.
These results demonstrate the excellent capabilities of LSTs for probing transient, fast-evolving VHE gamma-ray sources.

Tribunal:

President: Dr. Oscar Blanch Bigas

Secretary: Dr. Alicia López Oramas

Vocal: Dra. Roberta Zanin 

Suplents:

Dr. Abelardo Moralejo Olaizola 

Dra. Daniela Hadasch 

Directors: Dr. Pol Bordas Coma and Dr. Marc Ribó Gomis

Tutor: Dr. Alberto Manrique Oliva