Active Galactic Nuclei (AGN) are the nuclei of Galaxy which show energetic phenomena fromextremely compact volumes that can’t clearly and directly be attributed to stars. One distinctive characteristic of AGN is the fact that their energy flux varies along the whole electromagnetic spectrum, spanning a wide range of time scales from year to hours. Indeed, fluxvariability is often-used criterion for AGN detection (e.
g. Mushotzky 2004). The combinationof high luminosity and short variability time scale implies that the power of AGN is producedby phenomena more efficient in terms of energy release per unit mass than ordinary stellar pro-cesses (Fabian 1979). This basic argument leads to the hypothesis that massive black holes arepresent in the core of AGN.
Accretion of matter onto a black hole or extraction of its rotationalenergy can in fact yield high radiative efficiencies (Rees et al. 1982, Rees 1984). Within thewidely accepted canonical model, i.e. super massive black hole (SMBH) + accretion-disk (AD)+ dusty torus (DT) + relativistic jets, different regions of the active nucleus are thought tocontribute to the power emitted at different frequencies along the spectral energy distribution(SED).
Active Galactic Nuclei (AGN) classified into two largest subclasses are Seyfert galaxies andQuasars. Seyfert galaxies are the subclass of AGN hosts defined as a galaxies with bright,star-like nuclei that have relatively broad emission lines covering a wide range of ionization, inwhich host galaxy are clearly detectable. It become obvious that not all spectra from Seyfertgalaxies look same, so they are sub-classified into two types according to standard spectralclassification of Khachikian & Weedman (1974).
They differ by presence or absence of broadbase on the permitted emission lines. Type 1 Seyfert have both broad-line region (BLR) andnarrow-line region (NLR) present in their spectra while Type 2 Seyfert have only narrow-lineregion (NLR) present in their spectra. Narrow-line Seyfert 1 (NLSy 1) galaxies are special classof lower-luminosity AGN, defined by Osterbrock & Pogge (1985). They show width of narrowoptical balmer emission line FWHM(H?)< 2000 km s ?1 (Osterbrock et al.
1985, Goodrichet al. 1989), flux ratio of OIII ?5007 /H?< 3 (Shuder-Osterbrock et al. 1981), (however,exceptions are possible if they have strong Fe VII and Fe X lines), strong permitted optical/UV Feii emission lines (Boroson-Green et al. 1992, Grupe et al. 1999), steep soft X-rayspectra (Wang et al. 1996, Boller et al.
1996), rapid X-ray flux variability (Leighly et al.1999, Komossa-Meerschweinchen et al. 2000), and rapid optical flux variability (Miller et al.2000). They also show the radio-loud/radio-quite dichotomy, however, the radio-loud fractionof NLSy1 galaxies are about 7 per cent (Zhou et al.
2003), smaller than the fraction of 15 percent known in the population of quasars (Urry and Padovani 1995). Among the general AGN classes, many sub-classes such as NLSy1 galaxies display very peculiar properties. The interpretation of such peculiarity remain incomplete due to lack of multi-wavelength observation and/or due to the small number statistics. With the advent of large spectroscopic survey such as SDSS their sample sizes have now been increased substantially, and need to be analyse systematically using the recently launched multi-wavelength space mission like ASTROSAT inconjunction to the ground based observation. This form the main motivational of this thesisby focusing mainly on NLSy1 subclass of AGN, on key problems as listed in brief below.