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Peer-reviewedCC BY 4.0Monthly Notices of the Royal Astronomical Society

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

  • Krishan Chand1,2
  • Gopal-Krishna3
  • Ashutosh Tomar4,5
  • Vibhore Negi6
  • Hum Chand1
  • Suvendu Rakshit4
6 affiliations
  1. 1Central University of Himachal Pradesh (CUHP) Department of Physics and Astronomical Science, , Dharamshala 176215 ,
  2. 2Manipal Centre for Natural Sciences, Manipal Academy of Higher Education , Manipal ,
  3. 3UM-DAE Centre for Excellence in Basic Sciences, Vidyanagari , Mumbai 400098 ,
  4. 4Aryabhatta Research Institute of Observational Sciences (ARIES), Manora Peak , Nainital 263002 ,
  5. 5Mahatma Jyotiba Phule Rohilkhand University Department of Applied Physics/Physics, , Bareilly 243006 ,
  6. 6Kavli Institute for Astronomy and Astrophysics, Peking University , Beijing, 100871 ,
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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 (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.

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Cite

BibTeXRIS
BibTeX
@article{chand2026intranight,
  title = {{Intranight optical monitoring of eight quasars with superluminal jets and low optical polarization: Probing the mechanism of rapid flux variability}},
  author = {Krishan Chand and Gopal-Krishna and Ashutosh Tomar and Vibhore Negi and Hum Chand and Suvendu Rakshit},
  journal = {Monthly Notices of the Royal Astronomical Society},
  year = {2026},
  month = nov,
  doi = {10.1093/mnras/stag2086},
  url = {https://academic.oup.com/mnras/advance-article/doi/10.1093/mnras/stag2086/9011263},
  note = {CC BY 4.0},
}
RIS
TY  - JOUR
TI  - Intranight optical monitoring of eight quasars with superluminal jets and low optical polarization: Probing the mechanism of rapid flux variability
AU  - Krishan Chand
AU  - Gopal-Krishna
AU  - Ashutosh Tomar
AU  - Vibhore Negi
AU  - Hum Chand
AU  - Suvendu Rakshit
PY  - 2026
DA  - 2026/11/03
JO  - Monthly Notices of the Royal Astronomical Society
DO  - 10.1093/mnras/stag2086
UR  - https://academic.oup.com/mnras/advance-article/doi/10.1093/mnras/stag2086/9011263
AB  - 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.
SN  - 0035-8711
SN  - 1365-2966
ER  -