Probing Superluminous Supernovae with X-shooter Spectroscopy : Identifying pre-explosion eruptions and investigating spectral diversity

Wide-field time-domain surveys have become essential tools for discovering and characterizing astrophysical transients, particularly supernovae (SNe). These explosive events mark the terminal stages of stellar evolution. Stars spend the majority of their lives fusing hydrogen in their cores, but in the most massive stars, those exceeding roughly 8-10 solar masses, the nuclear burning progresses through advanced stages until an iron core is formed. Unable to support itself against gravity, this core implodes, triggering a core-collapse supernova (CCSN), violently expelling the outer layers of the star. Among CCSNe, superluminous supernovae (SLSNe) shine as some of the most extraordinary explosions in the Universe, outshining typical events by up to two orders of magnitude. Their extreme luminosities, extended light curves, and unusual spectra suggest massive progenitors and possibly exotic powering mechanisms. Yet, even within this rare class, significant diversity exists: some SLSNe exhibit spectral features that hint at unusual chemical compositions, while others show clear signs of eruptive mass loss at the onset of explosion. These events raise important questions about the late stages of stellar evolution, the variety of SLSN progenitors, and the physical mechanisms by which massive stars shed their outer layers in their final moments. This thesis addresses these questions in two main components. The first part (Paper I) explores the spectral diversity of SLSNe, revealing how subtle differences in their observed features reflect variations in progenitor systems and/or powering mechanisms. The second part (Paper II and Paper III) probes pre-explosion mass loss, uncovering how some progenitors undergo eruptions shortly before core collapse. Using a high-quality spectroscopic sample and advanced modeling techniques, this work demonstrates that the observed diversity in SLSNe is a direct window into the lives of massive stars, offering new insights into the final moments of the stellar evolution.