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.
RELATIVISTIC JETS: ACCELERATION, DISSIPATION AND INTERACTIONS WITH AMBIENT GAS\NCOLLIMATED, RELATIVISTIC OUTFLOWS, KNOWN AS RELATIVISTIC JETS, ORIGINATE FROM SUPERMASSIVE BLACK HOLES IN ACTIVE GALACTIC NUCLEI (AGN), SOLAR-MASS COMPACT OBJECTS IN X-RAY BIN
BLAZARS ARE A SUBCLASS OF ACTIVE GALACTIC NUCLEI WITH NON THERMAL, VARIABLE EMISSION EXTENDING OVER MOST OF THE ELECTROMAGNETICSPECTRUM, FROM RADIO UP TO GAMMA RAYS. THE BLAZAR EMISSION IS BELIEVED TO ORIGINATE IN RELATIVISTIC JETS EMERGING FROM SUPER M