The 2015–2017 Large EVPA Rotation in OJ 287: Dominant Propagating Component in a Helical Magnetic Field with Time-dependent Viewing Geometry

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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Journal
The Astrophysical Journal
Published
2026-11-06
DOI
https://doi.org/10.3847/1538-4357/aeb09b
Primary Topic
Astrophysics and Cosmic Phenomena
Type
article
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article

The 2015–2017 Large EVPA Rotation in OJ 287: Dominant Propagating Component in a Helical Magnetic Field with Time-dependent Viewing Geometry

Jeffrey Adam Hodgson, M. Tornikoski, Wanchaloem Khwammai, Seiji Kameno et al.
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The Astrophysical Journal
Astrophysics and Cosmic Phenomena
article

The 2015–2017 Large EVPA Rotation in OJ 287: Dominant Propagating Component in a Helical Magnetic Field with Time-dependent Viewing Geometry

Jeffrey Adam Hodgson, M. Tornikoski, Wanchaloem Khwammai, Seiji Kameno, Sascha Trippe, Minchul Kam, Anne Lähteenmäki, Do-Young Byun, Sang-Sung Lee, Hiroshi Nagai, Motoki Kino, Ilje Cho, Ioannis Myserlis, Taehyun Jung
article en

Abstract

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.

The Astrophysical JournalVol. 1011(2)
Kogakuin University (JP), Institute of Astronomy and Astrophysics, Academia Sinica (TW), Seoul National University (KR), The Graduate University for Advanced Studies, SOKENDAI (JP), Atacama Large Millimeter Submillimeter Array (CL), Korea Astronomy and Space Science Institute (KR), Sejong University (KR), Kyungpook National University (KR), Institut de Radioastronomie Millimétrique (FR), Instituto de Radioastronomía Milimétrica (ES), National Astronomical Observatory of Japan (JP), National Astronomical Research Institute of Thailand (TH), Aalto University (FI)
National Science Foundation, National Radio Astronomy Observatory, National Science Council, Aalto-Yliopisto, Korea Institute of Science and Technology Information, Korea Astronomy and Space Science Institute, National Research Foundation of Korea, National Astronomical Observatory of Japan, Korea Research Environment Open Network, Japan Society for the Promotion of Science, National Institutes of Natural Sciences
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Astrophysics and Cosmic Phenomena
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