Exploring Soft Gamma-Ray Bursts and Other Transient Phenomena

Gamma-Ray Bursts (GRBs) are the the most energetic electromagnetic emitters in the Universe allowing us to witness the high energy processes involved in the collapse of massive stars and the collisions of compact objects. The study of soft, low-luminosity GRBs is crucial for furthering our knowledge of the diversity of the GRB population. The SVOM and Einstein Probe (EP) missions launched in 2024, have increased our capability of detecting more soft GRBs in the future. In this thesis I present the multi-wavelength observational analysis of GRB 201015A, a soft, low-luminosity Swift GRB. This GRB has a uniquely late plateau in the X-ray afterglow, which can be explained with energy injection. The lack of a jet break observed in the afterglow indicates a large jet opening angle. The supernova associated with this GRB had a bright absolute magnitude of MR = −20.3 ± 0.7 mag. GRB 201015A is the 4th softest GRB in the Swift/BAT catalogue based on the photon index (Γ = 3.00+0.50 −0.42) of the initial prompt emission spectrum. The 4 GRBs with the softest photon index were compared with the general population of Swift GRBs, and they seemed to be consistent with the isotropic energy and spectral peak energy relations for long and short GRBs. However the spectral peak energy of GRB 201015A is below the BAT lower spectral energy threshold so only an upper limit can be determined. The SVOM ECLAIRs instrument has an energy range that reaches as low as the hard X-ray part of the electromagnetic spectrum, since it is designed to detect soft and high-redshift GRBs. I simulated SVOM observations to show that ECLAIRs can trigger on one of the softest Swift GRBs (GRB 150416A) out to very high redshift of z = 8.0. For GRB 201015A however, the maximum redshift at which it would trigger ECLAIRs is below 1, and simulated spectra did not provide a successful spectral peak measurement.

Other multi-wavelength transient phenomena such as active galactic nuclei (AGN), and X-ray binaries (XRBs) can provide insights into different types of energetic accreting systems. Cross-matching X-ray candidate transients from the Living Swift X-ray Point Source (LSXPS) catalogue with the optical observations from the GOTO telescope can provide additional evidence to confirm or classify transients, but it also involves challenges. So far I have found variability in the X-ray and optical observations of a supernova (Swift J111822.5-325014), and cataclysmic variable (Swift J073006.8-193709). The cross-matching tool I have created should help to identify new transients in the future using the X-ray and optical data.