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