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

Abstract

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 similar to 5.6 Earth's diameter, resulting in an angular resolution of similar to 530 mu as. With this unprecedented resolution at such a low frequency, the progressively bending jet structure of OJ 287 has been resolved up to similar 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 similar to 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 similar to 33 and similar to 11.5 GHz, and a turnover flux density similar to 4 and similar to 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 similar to 3.4 G for C1 and similar to 1.0 G for C2. The magnetic field strengths assuming equipartition arguments are also estimated as similar to 2.6 G and similar to 1.6 G, respectively. The integrated degree of linear polarization is found to be approximately similar to 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.