Owing to the high luminosity, quasars can be seen up to the very large distances, as a result their continuum and emission lines studies provides a unique tool to understand the driving parameters of central engine and the protogalaxy environment; and the absorption lines seen in their spectra give an important clue about the AGN feedback processes and the evolution of intergalactic medium, both of these features has been explored here in this thesis.
The thesis probe the nature of microvariability properties of broad absorption line quasars (BALQSOs), using both radio-loud and radio-quiet subtypes. It first aim to answer the observed high fraction of variability duty cycle (similar to the blazars) for BAL quasars subclass, which are primarily believed to be radio-quiet in nature. Enlarging the sample of radio-quiet BALQSO by a factor of three, we have shown, for the first time, that only ∼16 per cent of
BALQSOs show continuum variability. In a similar study, the extremely radio-loud BALQSOs have shown a similar duty cycle to that of their radio-quiet counterparts, resulting in supporting the models where the outflows, causing the broad absorption lines, may arise from the large variety of viewing angles from the jet axis or perhaps being closer to the disc plane. In a search for an elusive radio-quite counterparts of blazars, the newly discovered AGN class of weak-emission line quasars is found to be the potential candidate. Furthermore, we explore the hypothesis that, whether a weak jet component in radio-quiet AGNs is responsible for the origin of rapid small-scale optical variations or not? Using a sample of 83 radio-quiet quasars whose members has been
studied for continuum variability, we have ruled out the weak jet component model in radio-quiet quasars.
To probe the quasar environments, the thesis uses both the associated and intervening low ionization MgII absorption lines seen in AGN spectra. In the context the recent evidence of excess (factor ~2)
number density, dN/dz, of MgII absorption systems towards blazars over the value known for normal quasars, which has questioned the canonical view of intervening and associated absorption systems, we explore towards large sample of core-dominated and lobe-dominated quasars. One of our main result was that the associated absorbers can be seen even up to mildly relativistic speeds, i.e., several times more than the canonical limit of 5000 km/s adopted in the literature for associated absorbers. The dependence of magnetic filed on the presence and/or absence of the intervening absorbers at cosmic distances along quasar sightlines having polarized light has also been explored. This study has revealed a large contribution of the intervening absorber to the enhancement of residual rotation measure at around 21cm wavelength. Finally, to understand the key question of BALQSOs being overall X-ray weak (i.e., either intrinsic or absorbed), we have explored the CIV absorption line variability to probe the physical conditions near the central engine of a very rare class of X-ray-bright BALQSOs. We found a kinematic shift and strength
variability of the CIV BAL trough resulted in a largest kinematic shifts, exceeding by factor of about six as compared to the highest deceleration known till date.