Intra-night optical variability of core dominated radio quasars: the role of optical polarization

{Abridged} Rapid variations in optical flux are seen in many quasars and all blazars. The amount of variability in different classes of Active Galactic Nuclei has been studied extensively but many questions remain unanswered. We present the results of a long-term programme to investigate the intra-night optical variability (INOV) of powerful flat spectrum radio core-dominated quasars (CDQs), with a focus on probing the relationship of INOV to the degree of optical polarization. We observed a sample of 16 bright CDQs showing strong broad optical emission lines and consisting of both high and low optical polarization quasars (HPCDQs and LPCDQs). We employed ARIES, IIA, IGO telescopes, to carry out {\it R}-band monitoring on a total of 47 nights. Combining these INOV data with those taken from the literature, we were able to increase the sample size to 21 CDQs(12 LPCDQs and 9 HPCDQs) monitored on a total of 73 nights. As the existence of a prominent flat-spectrum radio core signifies that strong relativistic beaming is present in all these CDQs, the definitions of the two sets differ primarily in fractional optical polarization, the LPCDQs showing a very low median$ P_{op} \simeq$ 0.4 per cent. Our study yields an INOV duty cycle (DC) of $\sim$28 per cent for the LPCDQs and $\sim 68$ percent for HPCDQs. If only strong INOV with fractional amplitude above 3 per cent is considered, the corresponding DCs are $\sim$ 7 per cent and $\sim$ 40 per cent, respectively.From this strong contrast between the two classes of luminous, relativistically beamed quasars, it is apparent that relativistic beaming is normally not a sufficient condition for strong INOV and a high optical polarization is the other necessary condition.

💡 Research Summary

The authors present a systematic investigation of intra‑night optical variability (INOV) in flat‑spectrum, core‑dominated quasars (CDQs) with an emphasis on the role of optical polarization. Sixteen bright CDQs, all exhibiting strong broad emission lines and a prominent flat‑spectrum radio core (indicating relativistic beaming), were selected and divided into high‑polarization (HPCDQ, P_op > 3 %) and low‑polarization (LPCDQ, P_op < 1 %) subsamples. Using the ARIES, IIA, and IGO telescopes, the team performed R‑band monitoring over 47 nights, obtaining continuous light curves of 3–5 hours per night. By applying differential photometry, C‑statistics, and F‑tests, they identified significant INOV events and measured their fractional amplitudes (ψ).

To increase statistical power, the authors incorporated comparable data from the literature, expanding the sample to 21 CDQs (12 LPCDQs, 9 HPCDQs) observed on a total of 73 nights. The resulting INOV duty cycles (DC) were strikingly different: ~68 % for HPCDQs versus ~28 % for LPCDQs. When restricting the analysis to strong INOV (ψ > 3 %), the DC values become ~40 % for HPCDQs and only ~7 % for LPCDQs. This contrast demonstrates that relativistic beaming alone does not guarantee strong intra‑night variability; a high degree of optical polarization appears to be a necessary additional condition.

The authors interpret the findings in terms of jet physics: high optical polarization likely signals a more ordered magnetic field and the presence of shock‑driven or magnetic‑reconnection events that can produce rapid, high‑amplitude flux changes. Conversely, low‑polarization CDQs may have more turbulent or less efficiently beamed optical emission regions, resulting in weaker or absent INOV despite similar radio core properties.

Limitations include the modest sample size, uneven temporal coverage, and the fact that polarization measurements were not always contemporaneous with the INOV monitoring. The paper calls for future multi‑wavelength, simultaneous photometric and polarimetric campaigns to disentangle the interplay between jet dynamics, magnetic field geometry, and rapid variability. In summary, this work provides robust observational evidence that high optical polarization is a key prerequisite for strong intra‑night optical variability in relativistically beamed quasars, advancing our understanding of the micro‑physics governing active galactic nucleus jets.