Intranight Optical Variability of Radio-Quiet Weak Emission Line Quasars-III

TL;DR

This study investigates the intranight optical variability of radio-quiet weak-line quasars, aiming to identify potential radio-quiet BL Lac candidates and compare their variability characteristics with other active galactic nuclei classes.

Contribution

It provides new INOV observations of 15 radio-quiet weak-line quasars, highlighting two strong candidates for radio-quiet BL Lacs and comparing their variability with other AGN types.

Findings

RQWLQs show a higher INOV duty cycle than other radio-quiet quasars.

Two RQWLQs exhibit strong, blazar-like INOV, making them prime BL Lac candidates.

A lower detection threshold could reveal even higher INOV duty cycles.

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-line quasars' which are already designated as `high-confidence BL Lac candidate' and are also known to be radio-quiet. For 6 such radio-quiet weak-line quasars (RQWLQs), we present here new INOV observations taken in 11 sessions of duration >3 hours 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 hours. 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 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.