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We present the first very-long-baseline interferometric (VLBI) observationsof the blazar OJ287 carried out jointly with the Global Millimeter VLBI Array(GMVA) and the phased Atacama Large Millimeter/submillimeter Array (ALMA) at3.5 mm on April 2, 2017. Participation of phased-ALMA not only has improved theGMVA north-south resolution by a factor of ~3, but also has enabled fringedetection with signal-to-noise ratios up to 300 at baselines longer than 2G{\lambda}. The high sensitivity has motivated us to image the data with thenewly developed regularized maximum likelihood imaging methods, revealing theinnermost jet structure with unprecedentedly high angular resolution. Ourimages reveal a compact and twisted jet extending along the northwest directionwith two bends within the inner 200 {\mu}as that resembles a precessing jet inprojection. The component at the southeastern end shows a compact morphologyand high brightness temperature, and is identified as the VLBI core. Anextended jet feature that lies at ~200 {\mu}as northwest of the core shows aconical shape in both total and linearly polarized intensity, and a bimodaldistribution of the linear polarization electric vector position angle. Wediscuss the nature of this feature by comparing our observations with modelsand simulations of oblique and recollimation shocks with various magnetic fieldconfigurations. Our high-fidelity images also enabled us to search for possiblejet features from the secondary supermassive black hole (SMBH) and test theSMBH binary hypothesis proposed for this source.

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