Abstract:
This is continuation of our programme to search for the elusive radio-quiet BL Lacs, by
carrying out a systematic search for intranight optical variability (INOV) in a subset of ‘weak
emission line quasars’ which are already designated as ‘high-confidence BL Lac candidate’
and are also known to be radio quiet. For six such radio-quiet weak emission line quasars
(RQWLQs), we present here new INOV observations taken in 11 sessions of duration >3 h
each. Combining these data with our previously published INOV monitoring of RQWLQs in
19 sessions yields INOV observations for a set of 15 RQWLQs monitored in 30 sessions, each
lasting more than 3 h. The 30 differential light curves, thus obtained for the 15 RQWLQs, were
subjected to a statistical analysis using the F-test, and the deduced INOV characteristics of the
RQWLQs then compared with those published recently for several prominent active galactic
nucleus (AGN) classes, also applying the F-test. From our existing INOV observations, there
is a hint that RQWLQs in our sample show a significantly higher INOV duty cycle than radio-
quiet quasars and radio lobe-dominated quasars. Two sessions when we have detected strong
(blazar-like) INOV for RQWLQs are pointed out, and these two RQWLQs are therefore
the best known candidates for radio-quiet BL Lacs, deserving to be pursued. For a proper
comparison with the INOV properties already established for (brighter) members of several
prominent classes of AGN, a factor of 2–3 improvement in the INOV detection threshold
for the RQWLQs is needed and it would be very interesting to check if that would yield a
significantly higher estimate for INOV duty cycle than is found here.
