Active galactic nuclei (AGN) are the small regions of intensely energetic activity in the nucleiof many galaxies. Flux variation at all energy band over timescale of hours to years isone of the well known characteristics of AGN (Urry & Padovani1995). Extremely luminous,compactness and short variability time scale implies that the power of AGN is produced throughthe accretion of galactic material onto the super-massive black hole (Fabian 1979).According to the widely accepted unification model, AGN havea central super-massiveblack hole surrounded by a gaseous accretion disk. Fast moving Clouds of line-emitting gasaround this core surrounded by an obscuring torus or warped disk of gas and dust (e.g seeUlrich, Maraschi & Urry, 1997) give rise to broad and narrow emission line in the spectrum ofAGN. This has resulted in unified models of AGN, which unite two or more classes of objects,such as radio-loud, radio-quite, blazar etc classified based on the traditional observationalclassifications (Urry & Padovani 1995). Unification scheme proposed that these various classesof AGN are really a single type of physical object observed under different orientation and/oron different epoch of their evolution (Urry & Padovani 1995).The presence of prominent broad emission lines in the optical/UV spectrum is a hallmark ofthe Active Galactic Nuclei (AGN) designated as quasars. However, such lines can appear muchweaker for a class of AGN, called blazars, in which the optical/UV emission is dominated by thedoppler boosted non-thermal continuum from the relativistic jet and is therefore substantiallypolarized. Specifically, this weak line characterization holds for a subclass of blazars, called BLLac objects (BLOs), in contrast to the other blazar subclass, called highly polarized quasars(HPQs) which display the emission lines at a fairly strong level (e.g., Urry & Padovani 1995).Being jet dominated, both blazar sub-classes, HPQs and BLOsare radio loud in the sensethat the radio-to-optical flux density ratio R>10, where the radio and optical continuum fluxdensities refer to the rest-frame wavelengths of 6 cm and 2500?A, respectively (e.g., Kellermannet al. 1989; Stocke et al. 1992). But, whereas HPQs have an abundant population of weaklypolarized, radio-quiet counterparts (radio-quiet quasars: RQQs), the existence of radio-quietanalogs of BLOs (RQBLOs) continues to be an open question.Due to lack of possible candidates, few unsuccessful searches for radio-quiet BL Lacs werereported in the past (Januzzi et al. 1993; Londish et al. 2004). Recently large optical survey,such as Sloan Digital Sky survey (SDSS) (York et al. 2000) wasused by Collinge et al.(2005) and Anderson et al. (2007) to find candidates for radio-quiet BLOs. They termedsuch candidates as “Weak-Lines-Quasar” (WLQs). In this way, dozens of WLQs marked byabnormally weak broad emission lines (i.e, rest-frame EW<15.4?A for the Ly+NV emission3line complex (Diamond-Stanic 2009)) have been reported in the literature (Gopal-Krishna,Joshi, & Chand 2013). Since many of the WLQs are indeed found to be radio-quiet (e.g.,Plotkin 2010), they could potentially qualify as the elusive RQWLQs. It is a rare subclass ofquasars and its peculiarity lies in not exhibiting strong broad emission lines, the hallmark ofnormal quasars. Although over 300000 quasars have so far been found in the SDSS/BOSS,only a few dozen WLQs (or WLQ candidates) have been identifiedso far.2 A brief review of work already done in the fieldOver the past 15 years, examples of exotic radio-quiet quasars with intrinsically weak or absentbroad emission line regions (BELRs) have emerged from large-scale spectroscopic sky surveys.Such types of weak-lined quasars challenge both the standard orientation-based quasar uni-fication paradigm, and also quasar models that explicitly include factors beyond orientation(e.g., "disk+wind" BLR models). WLQs thus represent a new extreme of parameter space.An outstanding question is whether some relativistic plasma jets can even be radio-quiet (e.g.,see Gopal-Krishna et al. 2013)? Discovery of such jets wouldhave very basic implications forthe physics of AGN. By far, the best approach to find them wouldbe to search for radio-quietBL Lacs (RQBLs).These WLQs having weak emission line similar as seen for BL Lac objects (BLOs), remark-able differ in term of their radio loudness; the former being radio-quite (RQ) and the later asradio-louds. This is because, in contrast to BLOs (and much like RQQs), the radio-quietWLQs (RQWLQs) are found to exhibit low optical polarizationand mild optical continuumvariability on time-scales ranging from days to years. The reason for the abnormally weak lineemission in WLQs is yet to be fully understood, but the explanations proposed basically fallinto two categories. One possible cause of the abnormality is the high mass of the central blackhole which can result in an accretion disc too cold to emit strongly the ionizing UV photons,even when its optical output is high (Laor & Davis 2011, Plotkin 2010). Alternatively, thecovering factor of the broad-line region (BLR) in WLQs couldbe at least an order of magni-tude smaller compared to the normal QSOs (Nikolajuk & Walter2012). An extreme versionof this scenario is that in WLQs the accretion disc is relatively recently established and hencea significant BLR is yet to develop (Hryniewicz 2010, Liu & Zhang 2011). Conceivably, apoor BLR could also result from the weakness of the radiation-pressure driven wind when theAGN is operating at an exceptionally low accretion rate (Nicastro 2003, Elitzur & Ho 2009).While the above-mentioned limited empirical evidences andtheoretical scenarios are consistent4with the quasar interpretation of the bulk of the WLQ population, they do not rule out thepossibility of a small subset of the population being, in fact, the long sought radio-quiet BLOsin which optical emission arises predominantly from a relativistic jet of synchrotron radiation.Plotkin et al. (2010) selected a sample of 723 objects from SDSS Data release 7 (DR-7,Abazajian 2009), with weak spectral features selected based on their optical properties, basedon radio fluxes and limits from the FIRST/NVSS radio surveys,they subdivide their sampleinto 637 radio-loud BL Lac candidates and 86 radio-quiet weak-featured spectra. Opticalvariability and polarisation properties of these 86 radio-quiet WLQs have not been studiedyet. Therefore we derived a well defined sample out of these 86'high-confidence BL Laccandidates' for INOV monitoring and polarisation studies.Meusinger et al. (2014) performeda new search for quasars with weak emission lines in the spectroscopic data from the SDSS DR7.They visually inspected the 36 self-organising maps (Kohonen maps) from their previous paperfor nearly 105spectra classified as quasars by the SDSS spectroscopic pipeline and selecteda sample of?2500 WLQ candidates. After the thorough individual analysis of all selectedspectra we created a final sample of 365 WLQs with mean redshift z = 1.50±0.45. We extracteda well define sample for INOV and spectral studies out of 365 WLQs. Optical monitoring ofthis sample was not reported in literature. Smith et al. (2007) & Heidt et al. (2010) havepolarisation measurement of 27 WLQs, they did not find any potential radio-quiet BL Lacbased on these polarisation studies. In their sample, they did not check for proper motion andlater they found there are many galactic sources. Diamond-stanic et al. (2009) reported NIRspectral energy distribution of 4 WLQs and compared it with normal QSOs. They found thattwo WLQs are fainter in IR band by 30-40%. We have a large and well selected sample ofWLQs, which will be very fruitful for polarisation and NIR studies.3 ObjectivesThis PhD thesis entitled "On the nature of radio-quiet Weak Emission Line Quasars" is focusedon extraordinary quasars members?Radio-Quiet Weak Emission Line Quasars (RQWLQ).RQWLQ are recently discovered class of active galactic nuclei. Here, we aim to understandthe nature of this rare class of quasars (RQWLQs) by focusingon different properties :(i) The first will be to characterize the intranight optical variability (INOV) of RQWLQs. It iswell established that normal BLOs (which are always radio-loud) exhibit a distinctly strongerINOV, both in amplitude and duty cycle (DC), as compared to quasars, specially their radio-quiet majority, RQQs. From this it is evident that INOV properties can be a strong discrimi-5nator between blazars and other powerful AGN, both radio-loud and radio-quiet. The impetusbehind this new programme, therefore, is to characterize indetail the the INOV behavior ofRQWLQs using the Devsathal Fast Optical Telescope (DFOT) 1.3m telescope.(ii) The second part will consist of low resolution spectroscopic and polarisation investigations,such as to find out that what spectroscopic parameter(s) other than emission line strength, dodiffer among WLQs and other class of AGN. This will be crucial to pin-down the true mech-anism among various possibilities proposed for their weak emission lines phenomena. Withpolarisation analysis, we aim to test whether any of them show high polarisation like classicalBL Lac withP >3?4% to search for a bona-fide radio-quiet counter part of BL Lac. Therecently installed 3.6m telescope equipped with Aries-Devsathal Faint Object Spectrographand Camera (ADFOSC) will be ideal for such investigations using a bigger WLQs samples.(iii) In the third part, we will model the spectral energy distribution (SED) of these RQWLQs;especially focusing on IR/NIR and compare them with that of the composite SED of normalQSOs; either to confirm or refute the models of WLQs based on low covering factor of theBLR. 2.0m Himalayan Chandra Telescope (HCT) telescope equipped with TIFR Near InfraredSpectrometer and Imager (TRISPEC) will be ideal for observations.4 Noteworthy contribution in the field of proposed workWith our above proposed objective we can make following noteworthy contribution in theresearch field of AGN.•Based on INOV of RQWLQs:INOV property is a strong discriminator betweenblazars and other powerful AGN. For the first time we are reporting here duty cyclefor this interesting class of quasars (RQWLQs). The outcomewill be very important toknow whether RQWLQs are radio-quiet counterpart of Bl-Lac or not?•Spectral and polarisation properties :With this study of polarization properties,temporal properties at different time scale and spectral properties of RQWLQs, we willbe able to understand weather the mechanism of RQWLQs central engine resemble withnormal QSOs or with BL Lac/blazar like objects where relativistic jet emission plays animportant role ?•Based on BLR size of RQWLQs:The explanation of low covering factor of the BLRin WLQs would have additional consequence observed in the near infrared (NIR) band.6Diamond-Stanic et al. (2009 ApJ, 699, 782) mentioned that two weak line quasars arefainter in the IR band by 30-40%. To constrain more preciselythis discernment, we wishto use large and well selected sample of WLQs. Therefore, depending upon whether theaverage WLQs NIR/IR flux density is lower or not, will allow usto either confirm orrefute the model proposing small BLR in WLQs (hence the smalldusty torus causingreduction in IR emissivity) as a cause of their abnormally weak emission lines.5 Proposed methodologyOur groups have long experience of handling photometeric, polarisation and imaging datausing 1 – 4m telescope available all our the globe. Also spectral data reduction (both opticaland X-rays) our group has long working experience of high/low resolution spectroscopic dataof UVES/VLT, HARPS/ESO, EFOSC/ESO, IGO and XXM-newton.Continuum variabilityFor the continuum variability study, we will be using :•1m-Sampurnanad Telescope(ST), ARIES•1.3m-Devastal Fast Optical Telescope(DFOT), ARIES.•2m-Himalayan Chandra Telescope(HCT).•2m-Observatory Haute-Provence(OHP), France.Spectral & Polarisation propertiesThe spectral and polarisation properties of AGNs will be studied using data from 4m classTelescope facilities e.g.•3.6m-ESO telescope.•3.6m-Devsathal optical telescope (DOT), ARIES.Broad line region sizeThe NIR properties of RQWLQs will be studied using data from 2-4m class Telescope facilitiesavilable in India e.g.•2m-Himalayan Chandra Telescope(HCT).•3.6m-Devsathal optical telescope (DOT), ARIES.Archive Data:•Optical spectra from Sloan Digital Sky Survey (SDSS).76 Expected outcome of the proposed workWith our above proposed objective we expect following outcome:•Based on INOV of RQWLQs:Till now no one monitored the RQWLQs for theirintranight optical properties. Our well selected sample will for the first time determinethe duty cycle of RQWLQs and then we will compare it to other prominent class ofAGN reported in literature (Stalin 2004, Goyal 2012). As we know INOV property is astrong discriminator between blazars and other AGN classes. Blazars have a high dutycycle?40-50%. We also note here that if the RQWLQs do show a high dutycycle,this would indicates that RQWLQs have the same mechanism as blazars. Whereas, ifRQWLQs show low duty cycle then that would provide support tomechanism triggeringthe optical variability is similar to normal QSOs. This outcome will be very crucial toknow mechanism of RQWLQs central engine.•Spectral and polarimetric properties of RQWLQs:With this study of polarizationproperties and spectral properties of RQWLQs, we will be able to understand whetherthe mechanism of RQWLQs central engine resembles with normal QSOs or with BLLac/blazar like objects where relativistic jet emission plays an important role ? We willalso constrain their emission redshift, spectral slope with the help of spectral observation.Optical polarisation is one of the defining criteria for Bl Lacs objects. If any source inour sample show a high polarisation (P>3%) that would be an outstanding candidatefor the putative radio-quiet Bl Lacs.•Based on BLR size of RQWLQs:A large sample of RQWLQs will help us to eitherconfirm or refute the model proposing small BLR in WLQs (hencethe small dusty toruscausing reduction in IR emissivity) as a cause of their abnormally weak emission lines.