Intranight optical monitoring of eight quasars with superluminal jets and low optical polarization: Probing the mechanism of rapid flux variability

Intranight optical variability (INOV) of blazars whose emission arises predominantly from a relativistic jet, has been widely linked to a combination of two key observational diagnostics: the fractional optical polarization (Popt) and the Doppler boost arising from relativistic beaming of the jet’s emission. To discern which of these two factors plays the dominant role, we investigate INOV of a well-defined representative sample of 8 flat-spectrum radio quasars (FSRQs, which are ‘low-synchrotron-peaked’ blazars) whose jets exhibit a low degree of polarization (Popt < 3 %) but a large superluminal motion with cβapp = 5c − 31c (median = 10c). We report sensitive optical monitoring of these 8 ‘low-polarization superluminal quasars’ (LPSQs) using a 1.3-metre telescope, in 20 sessions of minimum 3-hour duration. These data were supplemented with similarly high-quality monitoring data/results available in the literature for this sample, thus adding another 19 sessions. Duty cycle (DC) of strong INOV (amplitude ψ > 5%) is found to be zero, applying the normally used Fη test, and ∼7% using the more powerful ‘enhanced F-test’. The small DC, despite the strong Doppler boost expected from the observed high superluminality, shows that strong INOV is much rarer among LPSQs, compared to high-polarization quasars, for which DC (ψ > 5%) is found to be around 30 %, even using the Fη test. This provides the first compelling evidence that optical polarization is the primary diagnostic of INOV, while relativistic beaming plays a supportive role. Possible implications for the proposed mechanisms for INOV are pointed out.