Unveiling the bent-jet structure and polarization of OJ 287 at 1.7 GHz with space VLBI

• Published in: Astronomy and astrophysics (Berlin) Vol. 683; p. A248
• Main Authors: Cho, Ilje, Gómez, José L., Lico, Rocco, Zhao, Guang-Yao, Traianou, Efthalia, Dahale, Rohan, Fuentes, Antonio, Toscano, Teresa, Foschi, Marianna, Kovalev, Yuri Y., Lobanov, Andrei, Pushkarev, Alexander B., Gurvits, Leonid I., Kim, Jae-Young, Lisakov, Mikhail, Voitsik, Petr, Myserlis, Ioannis, Pötzl, Felix, Ros, Eduardo
• Format: Journal Article
• Language: English
• Published: 01.03.2024
• ISSN: 0004-6361; 1432-0746
• DOI: 10.1051/0004-6361/202347157

We present total intensity and linear polarization images of OJ 287 at 1.68 GHz, obtained through space-based very long baseline interferometry (VLBI) observations with RadioAstron on April 16, 2016. The observations were conducted using a ground array consisting of the Very Long Baseline Array (VLBA) and the European VLBI Network (EVN). Ground-space fringes were detected with a maximum projected baseline length of ∼5.6 Earth’s diameter, resulting in an angular resolution of ∼530 μas. With this unprecedented resolution at such a low frequency, the progressively bending jet structure of OJ 287 has been resolved up to ∼10 parsec of the projected distance from the radio core. In comparison with close-in-time VLBI observations at 15, 43, 86 GHz from MOJAVE and VLBA-BU-BLAZAR monitoring projects, we obtain the spectral index map showing the opaque core and optically thin jet components. The optically thick core has a brightness temperature of ∼10 13  K, and is further resolved into two sub-components at higher frequencies labeled C1 and C2. These sub-components exhibit a transition from optically thick to thin, with a synchrotron self-absorption (SSA) turnover frequency estimated to be ∼33 and ∼11.5 GHz, and a turnover flux density ∼4 and ∼0.7 Jy, respectively. Assuming a Doppler boosting factor of 10, the SSA values provide the estimate of the magnetic field strengths from SSA of ∼3.4 G for C1 and ∼1.0 G for C2. The magnetic field strengths assuming equipartition arguments are also estimated as ∼2.6 G and ∼1.6 G, respectively. The integrated degree of linear polarization is found to be approximately ∼2.5%, with the electric vector position angle being well aligned with the local jet direction at the core region. This alignment suggests a predominant toroidal magnetic field, which is in agreement with the jet formation model that requires a helical magnetic field anchored to either the black hole ergosphere or the accretion disk. Further downstream, the jet seems to be predominantly threaded by a poloidal magnetic field.