Linear polarization structure of the BL Lacertae object OJ 287 at milliarcsecond resolution

The λ = 6 cm total intensity and linear polarization structures of the BL Lacertae object OJ 287 have been determined with an angular resolution of 2 x 9 milliarcseconds (mas) at two epochs a year apart. At each epoch the source can be modeled as a linear structure consisting of a core and two knots at position angle θ ≡ -115°. The core C was slightly polarized (m ≡ 4%), the inner knot K2 strongly polarized (up to 64%), and the outer knot K1 slightly polarized (∼3%). Superluminal motion for each knot, with an apparent velocity ∼3.3h-1c, is suggested by the polarization data. The polarizations of C and K2 changed markedly over the year between observations. The magnetic field in the jet appeared to be predominantly transverse to the jet; its direction did not change significantly over a year.