Searching for the signature of relativistic jets in stripped-envelope broad-line Ic supernovae

Resulting from the core-collapse of a massive star, broad-line Ic supernovae (Ic-BLs) are characterised by their smooth spectra with broad absorption features. Approximately one third of these Ic-BL supernovae are associated with a long gamma-ray burst (GRB). GRBs are highly luminous transients whose relativistic jets give rise to a brief period of high-energy emission followed by a long-lived, broadband emission. At low redshifts, the majority of long GRBs are associated with a Ic-BL supernova. The overall optical properties of ordinary Ic-BLs do not seem to differ from those of GRB-associated Ic-BLs. The discovery of an off-axis GRB associated with SN2020bvc supports the theory that relativistic viewing-angle effects may explain the lack of a GRB for some Ic-BLs; though late-time radio observations suggest that this cannot be the case for all Ic-BLs. This thesis investigates the connection between GRBs and Ic-BLs, searching for evidence of latent relativistic jets in Ic-BL supernovae through the lens of their velocity evolution. GRB-SN science requires a multiwavelength perspective, a perspective which is not provided by existing supernova or GRB repositories. The GRBSN Webtool has been developed to address this issue. By providing the multiwavelength data for every GRB-SN in a machine-readable format, and visualising the data, the GRBSN Webtool improves access to archival data and streamlines GRB-SN science. The trend of increasing GRB-SN detection rates is expected to continue, and the GRBSN Webtool is well suited to the expansion of the field. Computational studies have demonstrated that there may be some spectroscopic differences between Ic-BLs with and without GRBs, based on the viewing angle; while studies of their optical spectra suggest Ic-BLs expand more rapidly when associated with a GRB. This thesis presents the largest ever spectroscopic study of Ic-BL supernovae, presenting velocity measurements of over 800 photospheric spectra across 61 Ic-BLs and 13 GRB-SNe. The results of this analysis indicate that it is impossible to distinguish between GRB-associated Ic-BLs and ordinary Ic-BLs based purely on the spectral velocities, in contrast with previous studies. An investigation of the evolution of these velocities showed that many GRB-SNe and ordinary Ic-BLs follow a power-law decline in velocity, as expected for an expanding photosphere. However for some events there is evidence of broken power-law evolution which may indicate the presence of secondary velocity component. Jet activity leading to supernova asymmetry may be responsible for this; however, the extra velocity component is unlikely to be created by cocoon emission from the jet, due to the timescales involved. Given the minor discrepancy with previous studies of velocity, a review of methods for determining supernova velocities is performed, and recommendations for best practice are also presented.