Kinematic constraints on models of the BL Lacertea object OJ 287 from VLBI polarization observations

Recently published VLBI polarization observations of OJ 287 are discussed in terms of simple theoretical models. We show how kinematic constraints can be derived directly from the polarization data. For a simple ballistic model, in which the "pattern" and emitting material have the same velocity, the high degree of polarization combined with the comparatively small apparent speed constrain the Doppler shift to be less than 4.6, a smaller value than that required by X-ray and variability data. A model based on shocks in a purely relativistic jet is consistent with the available data and permits Doppler shifts as large as 7.5. In this model the shocks are propagating more slowly than the fluid.