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

TL;DR

This paper investigates the polarization structure of the core in the radio source 1803+784, proposing that recollimation shocks explain observed polarization features and contribute to jet brightening.

Contribution

It introduces an analytical model linking polarization patterns to recollimation shocks in astrophysical jets, supported by observational data.

Findings

Polarization pattern consistent with recollimation shocks

Model reproduces core polarization features at small viewing angles

Recollimation shocks may trigger jet brightening

Abstract

The extragalactic radio source 1803+784 is a BL Lac object that shows rapid variability and strong linear polarization. Very long baseline interferometry observations reveal that the core possesses a distinctive structure in polarized intensity with two maxima on axis and two minima symmetrically placed on either side. The approximately radial pattern of electric field polarization rods is reminiscent of the results obtained earlier by Cawthorne (2006) for conical shocks, but, individually, these do not reproduce the main features of the polarized intensity images. In numerical simulations and experiments, these shocks occur in pairs and help to stabilize jets as they adjust to changes in environment. Here, the polarization resulting from such structures is investigated using an approximate, analytical approach, by making some simple assumptions about the nature of the flow between two such shock waves. For fairly small viewing angles, it is found that a reasonable representation of the core polarization of 1803+784 can be obtained. The similarity between the observed and model polarization supports the view that the core structure in 1803+784 represents a recollimation shock, and that such shock waves may be responsible for the first disturbance and hence brightening of the quiescent flow in astrophysical jets.