The high-energy Universe remains enigmatic, with many astrophysical events still requiring complete understanding. Investigating these phenomena solely through electromagnetic radiation can pose significant challenges. Photons interact with matter and radiation during their journey from the sources to Earth, resulting in a degradation of the information they carry. Furthermore, even if the electromagnetic signal manages to reach Earth, its information can be ambiguous, making it difficult to decipher the underlying source physics. In contrast, neutrinos can be abundantly produced in these sources as a byproduct of particle acceleration. Due to their weakly interacting nature, they can travel undisturbed through space, giving access to environments that cannot be tested otherwise. This thesis aims to harness the potential of high-energy neutrinos to shed light on the enigmatic nature of some poorly understood transient events.
Our first focus is on long gamma-ray bursts originating from relativistic collimated jets born in the cataclysmic deaths of massive stars. They are the most powerful phenomena observed in the Cosmos with their exceptionally intense flashes in gamma rays, which can outshine entire galaxies for a fleeting period. Despite being discovered more than five decades ago, fundamental questions on the gamma-ray burst jet composition, energy dissipation, particle acceleration, and radiation mechanisms remain unanswered.
Different models exist, but no single model can explain the electromagnetic observations and multiple mechanisms may be at play across different gamma-ray bursts or even within a single burst event. The limited number of gamma-ray photons and the associated statistical challenges allow flexibility in fitting the same dataset with various input models. Hence, the information carried solely by electromagnetic emission has been insufficient to address all the open questions about jet workings. In this thesis, we investigate the potential of neutrinos in addressing these questions.
We then consider a class of extremely luminous supernovae that defy conventional emission mechanisms that describe core-collapse supernova emission: superluminous supernovae. One possible explanation for these exceptionally bright events invokes the interaction between energetic supernova ejecta and a very dense circumstellar medium formed by the progenitor star through intense winds and violent eruptions before the final supernova explosion. The mechanisms responsible for the substantial modification in the progenitor envelopes, which lead to significant loss of stellar mass, remain a challenging and active area of research. The neutrino signal, combined with the photon one, carries information about this ejected material structure, geometry, mass, and composition, which is crucial to pinpoint the type of progenitor involved and, ultimately, the underlying operating mechanisms.
Today, neutrino astronomy stands at a critical crossroads. The IceCube Neutrino Observatory has successfully confirmed the existence of a flux of high-energy cosmic neutrinos, and we have started seeing some significant associations with steady sources.
However, determining the origin of the bulk of these neutrinos has proven challenging because of the detector’s limited sensitivity. With the advent of new neutrino telescopes, such as IceCube Gen-2 and KM3Net, along with advancements in technology and data analysis techniques, we expect to significantly improve neutrinos’ detection capabilities. Given the positive premises, we believe that in the forthcoming years, these facilities will allow us to gain deeper insights into the transient sources investigated in this thesis.
In the following chapters, we introduce the mechanisms behind the acceleration of energetic particles in astrophysical environments and the production of high-energy neutrinos and electromagnetic radiation. Our primary focus is initially directed to the prompt emission phase of long-gamma ray bursts, aiming to improve our understanding of the nature of relativistic jets responsible for the gamma-ray emission. To this aim, we investigate neutrino production for various proposed models associated with prompt emission. To compare the different models, we employ up-to-date observations and simulation results. We then concentrate on some peculiar features observed during the afterglow emission of certain gamma-ray bursts, known as “optical jumps.” Assuming that these jumps arise from collisions between relativistic shells emitted due to late activity of the central engine, we investigate the prospects of detecting neutrinos and examine whether their detection can provide insights into the nature of these jumps.
In the second part of the thesis, we shift our attention to interaction-powered supernovae. Initially, we explore the interpretation of the transient event AT2019fdr as a superluminous supernova, aiming to determine whether the observed neutrino event IC200530A can be explained as originating from this superluminous supernova as opposed to the tidal disruption event interpretation. Motivated by our positive findings and the ever-growing detection rate of these rare transients, in the final part of the thesis, we explore the relation between high-energy neutrino production and photometric properties of interaction-powered supernovae, such as their optical peak luminosity and lightcurve rise time. The outcome of this investigation is crucial to guide and optimize upcoming targeted multimessenger searches of neutrinos from this class of transients.