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

Abstract

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 ($P_{opt}$) 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 ($P_{opt}$ < 3$\%$) but a large superluminal motion with c$β_{app}$ = 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 $F_η$ test, and $\sim 7\%$ 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 $F_η$ 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.