|Vineet Ojha (Aryabhatta Research Institute of Observational Sciences, Manora Peak, Nainital - 263002, India; Physical Research Laboratory, Astronomy & Astrophysics Division, Ahmedabad - 380009, India)
|In this thesis, we have done a detailed systematic study of the different characteristics of Narrow-line Seyfert 1 galaxies (NLSy1s, especially X-ray and gamma-ray detected sources) along with their comparison with other luminous classes of AGN. As the first part of it, we report the intra-night optical variability (INOV) study of 18 NLSy1s detected in the X-ray but not in gamma-ray (x$\textunderscore$NLSy1s), and seven sources detected in gamma-ray (gamma-ray). In the set of 18 x$\textunderscore$NLSy1s, the INOV duty cycle is found to be zero for the 13 radio-quiet members. However, the INOV duty cycle of 43%, found for the five radio-loud members is comparable to the 53% duty cycle of gamma-ray NLSy1s. Thus, it appears that the radio-loudness level indicating the presence of relativistic jet, is the prime factor behind the INOV detection, and the pattern of the high-energy radiation perhaps plays only a minor role. Secondly, based on 32 intra-night sessions ( > 3 hrs each), a blazar-like INOV duty cycle has been inferred from a sample of 14 gamma-ray NLSy1s discovered to date, even though they are highly-accreting AGNs. Furthermore, we also investigated the link of INOV detection of gamma-ray NLSy1s with the superluminal motion, and polarization parameters in addition to the radio-loudness level. Based on five > 3 hrs long intra-night monitoring sessions for each 3 gamma-ray NLSy1s for which the above parameters had already been measured, we found that the relativistic jet motion and/or small jet's angles to the observer's line of sight can be the robust diagnostic of the presence of INOV in NLSy1s in comparison to their radio-loudness level. Finally, we have done a detailed comparative systematic study using a sample of 221 NLSy1s in comparison to a redshift-matched sample of 154 BLSy1s based on their spectral properties in optical and X-ray bands.