The 2015–2017 Large EVPA Rotation in OJ 287: Dominant Propagating Component in a Helical Magnetic Field with Time-dependent Viewing Geometry
13 affiliations
- 1National Astronomical Research Institute of Thailand (Public Organization) 260 Moo 4 Donkaew
- 2Institute of Astronomy and Astrophysics
- 3Seoul National University
- 4National Astronomical Observatory of Japan
- 5Kogakuin University of Technology & Engineering
- 6The Graduate University for Advanced Studies
- 7Joint ALMA Observatory
- 8Kyungpook National University
- 9Korea Astronomy and Space Science Institute
- 10Institut de Radiostronomie Milimétrique
- 11Sejong University
- 12Aalto University Metsähovi Radio Observatory
- 13Aalto University Department of Electronics and Nanoengineering
Abstract
We present high-cadence, multifrequency monitoring of the blazar OJ 287 using Korean VLBI Network (KVN; 22–129 GHz), Atacama Large Millimeter Array (ALMA; 91.5–343.5 GHz), and Metsähovi (37 GHz), covering 2012–2023, together with published optical polarimetry. Within this decade-long dataset, an exceptionally large and smooth electric vector position angle (EVPA) rotation is observed only during 2015–2017. The millimeter-band EVPA rotates by more than ∼300 ∘ over ∼1.5 yr, with a comparably large and more rapidly evolving rotation observed at optical wavelengths, while remaining comparatively stable at other epochs. The rotation coincides with a sequence of strong radio flares from 37 to 343 GHz whose peak amplitudes increase toward 2017 March, when the rotation ends. Modeling the ALMA 91.5 GHz light curve yields variability Doppler factors of δ var ∼ 8–11 during the EVPA-rotation interval, with no systematic increase across the flare sequence. The absence of a monotonic change in δ var indicates that progressively enhanced relativistic beaming is unlikely to be the primary driver of the rising flare envelope. The confinement of the large EVPA rotation to this interval, together with the nearly constant δ var , indicates that the 2015–2017 event occurred when a single newly ejected disturbance temporarily dominated the polarized emission while propagating through structured inner-jet regions threaded by a helical magnetic field under modest jet-direction changes. Large EVPA rotations therefore arise intermittently as the jet orientation evolves, and are realized only when emission dominance and viewing geometry align favorably.
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BibTeX
@article{khwammai202620152017,
title = {{The 2015–2017 Large EVPA Rotation in OJ 287: Dominant Propagating Component in a Helical Magnetic Field with Time-dependent Viewing Geometry}},
author = {Wanchaloem Khwammai and Minchul Kam and Motoki Kino and Sascha Trippe and Hiroshi Nagai and Seiji Kameno and Ilje Cho and Ioannis Myserlis and Jeffrey Hodgson and Do-Young Byun and Sang-Sung Lee and Taehyun Jung and Anne Lähteenmäki and Merja Tornikoski},
journal = {The Astrophysical Journal},
year = {2026},
month = nov,
doi = {10.3847/1538-4357/aeb09b},
url = {https://iopscience.iop.org/article/10.3847/1538-4357/aeb09b},
note = {CC BY 4.0},
}
RIS
TY - JOUR
TI - The 2015–2017 Large EVPA Rotation in OJ 287: Dominant Propagating Component in a Helical Magnetic Field with Time-dependent Viewing Geometry
AU - Wanchaloem Khwammai
AU - Minchul Kam
AU - Motoki Kino
AU - Sascha Trippe
AU - Hiroshi Nagai
AU - Seiji Kameno
AU - Ilje Cho
AU - Ioannis Myserlis
AU - Jeffrey Hodgson
AU - Do-Young Byun
AU - Sang-Sung Lee
AU - Taehyun Jung
AU - Anne Lähteenmäki
AU - Merja Tornikoski
PY - 2026
DA - 2026/11/06
JO - The Astrophysical Journal
DO - 10.3847/1538-4357/aeb09b
UR - https://iopscience.iop.org/article/10.3847/1538-4357/aeb09b
AB - We present high-cadence, multifrequency monitoring of the blazar OJ 287 using Korean VLBI Network (KVN; 22–129 GHz), Atacama Large Millimeter Array (ALMA; 91.5–343.5 GHz), and Metsähovi (37 GHz), covering 2012–2023, together with published optical polarimetry. Within this decade-long dataset, an exceptionally large and smooth electric vector position angle (EVPA) rotation is observed only during 2015–2017. The millimeter-band EVPA rotates by more than ∼300 ∘ over ∼1.5 yr, with a comparably large and more rapidly evolving rotation observed at optical wavelengths, while remaining comparatively stable at other epochs. The rotation coincides with a sequence of strong radio flares from 37 to 343 GHz whose peak amplitudes increase toward 2017 March, when the rotation ends. Modeling the ALMA 91.5 GHz light curve yields variability Doppler factors of δ var ∼ 8–11 during the EVPA-rotation interval, with no systematic increase across the flare sequence. The absence of a monotonic change in δ var indicates that progressively enhanced relativistic beaming is unlikely to be the primary driver of the rising flare envelope. The confinement of the large EVPA rotation to this interval, together with the nearly constant δ var , indicates that the 2015–2017 event occurred when a single newly ejected disturbance temporarily dominated the polarized emission while propagating through structured inner-jet regions threaded by a helical magnetic field under modest jet-direction changes. Large EVPA rotations therefore arise intermittently as the jet orientation evolves, and are realized only when emission dominance and viewing geometry align favorably.
SN - 1538-4357
SN - 0004-637X
ER -