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The extragalactic radio source 1803+784 is a BL Lac object that shows rapidvariability and strong linear polarization. Very long baseline interferometryobservations reveal that the core possesses a distinctive structure inpolarized intensity with two maxima on axis and two minima symmetrically placedon either side. The approximately radial pattern of electric field polarizationrods is reminiscent of the results obtained earlier by Cawthorne (2006) forconical shocks, but, individually, these do not reproduce the main features ofthe polarized intensity images. In numerical simulations and experiments, theseshocks occur in pairs and help to stabilize jets as they adjust to changes inenvironment. Here, the polarization resulting from such structures isinvestigated using an approximate, analytical approach, by making some simpleassumptions about the nature of the flow between two such shock waves. Forfairly small viewing angles, it is found that a reasonable representation ofthe core polarization of 1803+784 can be obtained. The similarity between theobserved and model polarization supports the view that the core structure in1803+784 represents a recollimation shock, and that such shock waves may beresponsible for the first disturbance and hence brightening of the quiescentflow in astrophysical jets.

Polarization structure in the core of 1803+784: a signature of recollimation shocks?