1996A&A...312..727V

The structural evolution of the BL Lac object OJ 287 is studied with milliarcsecond resolution by using 8.4GHz geodetic Very Long Baseline Interferometry (VLBI) observations acquired by the Crustal Dynamics Program. OJ 287 has been mapped at ten epochs separated by intervals of a few months between 1985.4 and 1988.1. Our maps provide evidence that the superluminal jet component K3, ejected during the 1983-1984 optical outburst, has decelerated by mid-1986 and reaccelerated afterwards. We explain this peculiar motion by a projection effect due to a helical morphology of the jet. Our estimated helical model corresponds to a helix with a pitch angle of 20deg on a narrow cone of half opening angle 3.4deg, and central axis inclined at 17deg relative to the line of sight. Ejection of VLBI components appears to occur simultaneously with optical outbursts, but also at mid-time between them. This finding is consistent with the supermassive binary black hole model previously proposed to explain those periodical outbursts. Comparison of our helical fit with hydrodynamical models provides estimates of the radius, opening angle, and Mach number of the jet, and shows that the orbital 9.0yr period of the binary black hole system is adequate to drive the helical perturbation if the jet propagates in a hot ambient medium.