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 OJ287 at 1.68GHz, obtained through space-based VLBI observations with RadioAstron on April 16, 2016. The observations were conducted using a ground array consisting of the VLBA and the 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 uas. With this unprecedented resolution at such a low frequency, the progressively bending jet structure of OJ287 has been resolved up to 10 pc 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 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.