Unravelling the Innermost Jet Structure of OJ 287 with the First GMVA+ALMA Observations

TL;DR

This paper presents the first high-resolution VLBI observations of blazar OJ 287 using GMVA and ALMA, revealing detailed jet structures and testing the SMBH binary hypothesis.

Contribution

It introduces new high-sensitivity imaging of OJ 287's innermost jet with unprecedented resolution, utilizing combined GMVA+ALMA data and advanced imaging techniques.

Findings

Revealed a compact, twisted jet with bends within 200 μas.

Identified a conical extended jet feature with polarization characteristics.

Provided evidence supporting or refuting the SMBH binary hypothesis.

Abstract

We present the first very-long-baseline interferometric (VLBI) observations of the blazar OJ287 carried out jointly with the Global Millimeter VLBI Array (GMVA) and the phased Atacama Large Millimeter/submillimeter Array (ALMA) at 3.5 mm on April 2, 2017. Participation of phased-ALMA not only has improved the GMVA north-south resolution by a factor of ~3, but also has enabled fringe detection with signal-to-noise ratios up to 300 at baselines longer than 2 G{\lambda}. The high sensitivity has motivated us to image the data with the newly developed regularized maximum likelihood imaging methods, revealing the innermost jet structure with unprecedentedly high angular resolution. Our images reveal a compact and twisted jet extending along the northwest direction with two bends within the inner 200 {\mu}as that resembles a precessing jet in projection. The component at the southeastern end shows a compact morphology and high brightness temperature, and is identified as the VLBI core. An extended jet feature that lies at ~200 {\mu}as northwest of the core shows a conical shape in both total and linearly polarized intensity, and a bimodal distribution of the linear polarization electric vector position angle. We discuss the nature of this feature by comparing our observations with models and simulations of oblique and recollimation shocks with various magnetic field configurations. Our high-fidelity images also enabled us to search for possible jet features from the secondary supermassive black hole (SMBH) and test the SMBH binary hypothesis proposed for this source.