We present Very Long Baseline Array observations of the radio galaxy ( catalog 3C~120 ) at 5 , 8 , 12 , and 15 GHz designed to study a peculiar stationary jet feature ( hereafter C80 ) located \sim 80 mas from the core , which was previously shown to display a brightness temperature \sim 600 times lager than expected at such distances .
The high sensitivity of the images – obtained between December 2009 and June 2010 – has revealed that C80 corresponds to the eastern flux density peak of an arc of emission ( hereafter A80 ) , downstream of which extends a large ( \sim 20 mas in size ) bubble-like structure that resembles an inverted bow shock .
The linearly polarized emission closely follows that of the total intensity in A80 , with the electric vector position angle distributed nearly perpendicular to the arc-shaped structure .
Despite the stationary nature of C80/A80 , superluminal components with speeds up to 3 \pm 1 c have been detected downstream from its position , resembling the behavior observed in the HST-1 emission complex in ( catalog M87 ) .
The total and polarized emission of the C80/A80 structure , its lack of motion , and brightness temperature excess are best reproduced by a model based on synchrotron emission from a conical shock with cone opening angle \eta = 10 ^ { \circ } , jet viewing angle \theta = 16 ^ { \circ } , a completely tangled upstream magnetic field , and upstream Lorentz factor \gamma _ { u } = 8.4 .
The good agreement between our observations and numerical modeling leads us to conclude that the peculiar feature associated with C80/A80 corresponds to a conical recollimation shock in the jet of ( catalog 3C~120 ) located at a de-projected distance of \sim 190 pc downstream from the nucleus .