Exploring transients in the X-ray and the ultraviolet

Transient astronomical sources can be used to probe extreme physical processes. In this thesis, I explore transient phenomenon at the X-ray and ultraviolet wavelengths. The X-ray studies probe low-mass X-ray binaries, which are constituted of a neutron star or black hole along with a low-mass companion star. In transient neutron star low-mass X-ray binaries, the neutron star crust may be heated by accretion onto its surface during an outburst. Once the outburst ceases, the crust begins to cool. The observed cooling behaviour is used to infer properties of the dense matter present in neutron star crusts (which hosts matter at densities much higher than that present on Earth). Accretion is a ubiquitous process in the universe. In my thesis, I have studied accretion physics using low-mass X-ray binaries hosting both neutron stars and black holes. This involves the study of unusual accretion phenomena as well as studies to probe the connection between the accretion inflow and the outflow. Several explosive transients (e.g., supernovae and neutron star mergers) primarily emit in the ultraviolet, making ultraviolet an optimal wavelength to find as well as study such transients. Despite this, they are very poorly studied in the ultraviolet regime. In my thesis, I present the automatic transient search pipeline I have developed which examines the ultraviolet data obtained using the Neil Gehrels Swift Observatory and applied it to the observations of the Small Magellanic Cloud.