Some of the brightest and most energetic events in the Universe are associated with the death of stars. These stellar deaths power transient electromagnetic emission which are routinely observed on Earth. This dissertation presents our research on various such transients. Its topics includes, supernova remnants, kilonovae, gamma-ray bursts (GRBs): The "long'' type produced from core-collapse supernovae and the "short'' type associated with neutron star merger events. It also focuses on the disruption of stars by the tidal forces of supermassive black holes i.e., tidal disruption events (TDEs). We model the emission from these transients and compare them to observations in order to draw a number of conclusions and make predictions for future detections. For example, we find that the non-thermal emission from supernovae and kilonovae associated with GRBs can produce long term emission which may be detected as a re-brightening in the overall emission. The sharp cut off observed in some TDE flares can be caused by a pre-existing accretion disk present around a supermassive black hole, which is expected in active galactic nuclei. Our work successfully predicted the nature of the very first electromagnetic detection from a neutron star merger, and was able to reproduce the emission that had been observed for more than one hundred days after the merger. This dissertation also provides frameworks on how the observable features of these transients can be leveraged to probe the properties of the progenitor system and their environment. <br>

Blazars are a type of Active Galaxy that emit strong astrophysical jets. The association of a HE gamma-ray flare from the blazar TXS 0506+056 to the IceCube-170922A neutrino event in 2017, opened the possibility to a link between these two events. In this thesis, we will look at the HE gamma-ray behavior of TXS 0506+056 using data obtained from the Fermi-LAT by taking into account the other set of neutrino events associated with this source from 2014-2015. We will investigate whether both neutrino events present with comparable HE gamma-ray behavior by analyzing the lightcurves and the spectra for a quiet state, the 2014-2015 period, and the flare centered around the neutrino event from 2017. The results of the analysis performed in this thesis show no strong indication of a change in the gamma-ray behaviour in these potential neutrino detections.

The origin of Ultra-High-Energy Cosmic Rays (UHECRs) is still an open question in Astrophysics. The scope of this thesis is the study of the termination shocks in the jets of radiogalaxies 3C 105, 3C 227, and 3C 445 as cosmic rays accelerators. We assume the diffusive shock acceleration (DSA) and that the maximum energy is determined by the escape of cosmic rays downstream of the shock due to the magnetization. We find maximum achievable energy of non-thermal electrons and protons, which appeared to be only~TeV. Therefore the reverse shocks of AGNs' jets are not the sources of UHECRs. We also implement the model, where the scattering centers for the DSA emerge from the non- resonant hybrid instabilities in plasma. We take into account the results from the numerical simulations and observations that indicate the amplification of the magnetic field by orders of magnitude, and we obtain results that qualitatively fit the simulation outcome.