Searching for High-Energy Neutrinos from Transient Sources with IceCube

The goal of this thesis is to probe astrophysical transients as neutrino sources. Bright flares from enhanced accretion onto supermassive black holes (SMBHs) were reported to be correlated with IceCube's high-energy (>100 TeV) neutrino alerts. They are characterized by bright, transient infrared emission, called dust echos. For the first time, this work probes the general population of accretion flares using the full IceCube muon neutrino sample with a stacking analysis. The non-detection constrains their contribution to the diffuse neutrino flux to below around 7.1% for a spectral index of 2.5. Neutrino production through interaction with X-ray photons from the accretion disk is excluded, and interaction with optical/UV photons is disfavoured. Although high-energy neutrino production through proton interaction with the infrared photons can still not be excluded, model parameters are constrained to below optimistic values. An improved method for calibration of the localization uncertainty for IceCube's high-energy neutrino alerts is also developed, applicable to through-going muon events, and for the first time also for events with unusual energy deposition patterns. Results for the neutrinos reported in coincidence with accretion flares indicate that the corresponding uncertainty contours need robust calibration. To enable a more sensitive search for neutrinos from accretion flares with bright dust echos, a catalog of dust-echo-like flares is compiled. This is the widest and deepest such catalog to date, increasing the sample size compared to the previous analysis by a factor of ten. Assuming the sample is dominated by TDEs, it is the first one to provide enough statistics at higher redshifts to probe the evolution of their rate. At least for black hole masses that enable the disruption of a solar mass star, the evolution is consistent with expectations from number density evolutions of SMBHs.