MONITORING OF THE VLBI RADIO STRUCTURE OF THE BL-LACERTAE OBJECT OJ-287 FROM GEODETIC DATA

The structural evolution of the BL Lac object OJ 287 is studied with m illiarcsecond resolution by using 8.4 GHz geodetic Very Long Baseline Interferometry (VLBI) observations acquired by the Crustal Dynamics Pr ogram. 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 optic al outburst, has decelerated by mid-1986 and reaccelerated afterwards. We explain this peculiar motion by a projection effect due to a helic al morphology of the jet. Our estimated helical model corresponds to a helix with a pitch angle of 20 degrees on a narrow cone of half openi ng angle 3.4 degrees, and central axis inclined at 17 degrees relative to the line of sight. Ejection of VLBI components appears to occur si multaneously with optical outbursts, but also at mid-time between them . This finding is consistent with the supermassive binary black hole m odel previously proposed to explain those periodical outbursts. Compar ison of our helical fit with hydrodynamical models provides estimates of the radius, opening angle, and Mach number of the jet, and shows th at the orbital 9.0 yr period of the binary black hole system is adequa te to drive the helical perturbation if the jet propagates in a hot am bient medium.