n old, outstanding question in extragalactic astrophysics is about
the existence of a tiny subset of BL Lacs whose flux is relatively
insignificant at radio wavelengths, akin to quasars whose vast
majority is actually radio-quiet (broadly meaning that the radio flux
density does not exceed the optical flux density). Discovery of even a
minuscule set of bona-fide radio-quiet BL Lac among them would have
fundamental implications for the current understanding of jet
physics. BL Lac objects represent a numerically small but highly
active subclass of relatively powerful active galactic nuclei
population. They are characterized by strong and rapid variability of
both continuum and polarized emission, commonly attributed to the
dominance of a relativistically boosted jet of synchrotron radiation,
emanating from the core of the host galaxy. Recently, their findings
got a major boost with the advent of large-area optical
imaging/spectroscopic surveys, such as the Sloan Digital Sky Survey
and the Two-Degree Field QSO Redshift Survey. Indeed, these surveys
picked up hundreds of star-like objects, mostly at high redshifts, z
>~ 2, whose optical spectra are largely featureless, termed as
radio-quiet weak emission line quasars (RQWLQs). Therefore it seems
worthwhile to investigate a well-selected sample of optically selected
WLQs whose members have (i) a radio-loudness parameter (R) < 10, to
ensure radio-quietness and (ii) a secure redshift measurement, or at
least a proper motion consistent with zero. This Ph.D. thesis is aimed
to understand the nature of this rare class of quasars, Radio-Quiet
Weak Emission Line Quasars (RQWLQs) by studying different properties.
In our INOV analysis, we have mainly carried out the monitoring from
ARIES 1.3m telescope, supplemented for few nights by two other 2m
telescopes. About a sample of about three dozen of RQWLQs is monitored
for about 5 dozen of the session with duration 3-4hrs. We found that
estimated INOV duty cycle of the RQWLQs is about 3-5% which appears
inconsistent with BL Lacs contrary to the similarity with normal
QSOs. However, the observed INOV events (which occurred in just two of
the sessions) are strong (with a fractional variability amplitude > 10
percent), leading to a high confidence candidate for the putative
radio-quiet Bl lac object. Similarly based on our modest sample of 6
RQWLQs studied for their polarimetric properties, only two are found
to have p > 1%, which again is too modest to justify a (radio-quiet)
BL Lac classification. In our spectral analysis, we carry out a
statistical comparison of the optical spectral index, for a set of 40
RQWLQs with redshift-luminosity matched control sample of 800 QSOs and
an equivalent sample of 120 blazars. The spectral index distribution
of RQWLQs is found to differ, at a high significance level, from that
of blazars and is consistent with that of the ordinary QSOs. In
conclusion, our findings are consistent with the common view that the
central engine in RQWLQs, as a population, is akin to that operating
in normal QSOs and the primary differences between them might be
related to differences in the BLR, and they may be QSO population in
their early phase of evolution.