The Sloan Digital Sky Survey (SDSS) and Two-Degree Field QSO Redshift Survey (2QZ) have
revealed a population of Active Galactic Nuclei (AGNs) with peculiar spectral characteristics.
These objects have featureless optical discovery spectra resembling BL Lac objects. However, they
are significantly quiet in radio and X-ray bands compared to classical BL Lacs. These objects
could therefore belong to the hitherto unrecognised population of radio-quiet BL Lacs or radio-
quiet quasars without a Broad Line Region (BLR). How such objects fit into the AGN unification
picture is an open question. Polarization and variability are the two main characteristic properties
of BL Lac objects. Last year, I was involved in two such project, first one is polarisation and
spectroscopy study of radio-quiet BL Lacs.
For this we carried out spectral and polarisation measurements of a sample of 19 such RQWLQs.
Out of them only 9 sources show non-significant proper motion, and all of them show percentage
polarisation (P) less than 1%, except two sources J142505.59+035336.2 and J154515.77+003235.2
with highest polarization 1.59 ± 0.53%. This small percentage polarisation of RQWLQs seems
too low for them to be the radio-quite analogue of Bl-lac objects. We also report a statistical
comparative study of the spectral slope, temporal variation of the continuum flux at different
time scales using a sample of 45 RQWLQs with 900 redshift-luminosity matched control sample of
QSOs and 120 blazar collected from the literature. The structure function analysis shows that the
mechanism triggering the optical variability in RQWLQs seems similar to that in QSOs and unlike
blazar which are generally highly variable at all time scales. Similarly, the spectral slop distribution
of RQWLQs differ at high significance from that of blazars, suggesting that the mechanism of
RQWLQs central engine might be resembling more with that of normal QSOs, perhaps with less
developed BLR as a cause of their weak emission lines; unlike the Bl-lac/blazar objects whose
continuum emission are dominated mainly by their relativistic jet. The paper has been written and
submission is expected in next two three weeks.