Death of massive stars in a multimessenger perspective

This thesis focusses on massive stars that, after exploding as supernovae (SNe), show evidences of strong interaction between the SN ejecta and a dense circumstellar medium (CSM), which is expected to be ejected by the progenitor star during eruptive mass-loss episodes shortly before the terminal explosion. In a multimessenger context, these transients are suspected to be among the possible sources of high energy (HE) neutrinos in the TeV to PeV range. After a brief introduction to the multimessenger scenario, in Chap. 1 we introduce the different explosion mechanisms of SNe, the physics of interaction and of neutrino production. Indeed, HE neutrinos are though to be produced in shocked regions of the CSM that are excited by the interaction with the fast-expanding SN ejecta, although their rate of production may not be high. We then describe different types of interacting SNe, with a particular focus on their spectrophotometric characteristics. In Chap. 2, we describe our results of the search of a possible correlation between HE neutrinos detected by the IceCube neutrino detectors and SNe. In this context we used three different approaches: • a search for new transients inside the neutrino positional errorbox, • follow-up observations of interacting SNe inside the neutrino errorbox, • a statistical analysis of the cross match between SN and neutrino catalogues. The result of all these approaches was inconclusive, as we did not find evidences of a link between HE neutrinos and interacting SNe. However, this does not mean that interacting SNe cannot produce HE neutrinos, since results on a global population do not exclude the chance of finding a direct link between a transient and an individual neutrino event, as occurred with blazars and tidal disruption events. Therefore, in the next Chapters we investigated the properties of strongly-interacting SN candidates in order to characterise the mass and energy of the ejecta, as well ass the mass and radius of the CSM. In Chap. 3, we describe the sample of strongly-interacting SN candidates we selected for follow- up. We show the data we collected through our active proposals and collaborations, describe the data reduction processes, and present the selection criteria that led us to the target selection. In Chap. 4, we show the analysis we performed on one of the selected candidates, SN 2020faa. We demonstrated, through the application of a toy model, that the light curve of this SN is in agreement with being powered by hidden interaction and we found that the interaction energy and CSM mass are compatible with HE neutrino production. This is particularly interesting because it provides a new class of transients that are possible HE neutrino counterparts that had been overlooked before. In Chap. 5, we examine a sample of more typical strongly-interacting SNe, for which the interaction is exposed. After a thorough analysis of their spectrophotometric characteristics, in the last part of the Chapter we focus on the parameters that could suggest HE neutrino production. In particular, we derive the mass and radius of the CSM and the kinetic energy of the ejecta and compare them to theoretical models. We find that these transients are indeed compatible with the production of HE neutrinos, albeit they remain at the lower end of the interecting region. Finally, we conclude in Chap. 6 briefly describing the new, upcoming facilities that on the one hand will increase the neutrino statistics and improve the localisation and on the other hand will ensure a complete follow-up of strongly-interacting SNe, in order to improve our understanding of these transients and their multimessenger connections.