Resolving the jet in Cygnus A

TL;DR

This study uses 43GHz VLBI observations to investigate the inner structure and dynamics of the Cygnus A jet and counter-jet, revealing new features and jet collimation details.

Contribution

First high-frequency VLBI imaging of Cygnus A's inner jet region, identifying a potential new counter-jet component and detailed jet collimation behavior.

Findings

Detected a gap between jets indicating possible new component or acceleration region

Resolved jet structures beyond 0.5pc facilitating stratification studies

Observed jet collimation from ~10° to ~3° within the first parsec

Abstract

Our previous studies revealed a good kinematic model for the jet of Cygnus A, but the counter-jet speed is still not well constrained. The central engine and part of the counter-jet of Cyg A are likely to be obscured by free-free absorbing material, presumably a thick torus. At mm-wavelengths, the absorber becomes optically thin, which provides a more detailed view into the inner nuclear region. Knowing the speed of jet and counter-jet and their flux density ratio allows to determine the jet Lorentz factors and orientation. Therefore we started to monitor Cyg A with global VLBI at 43GHz in Oct. 2007. Our first epoch reveals a previously unseen gap between both jets. This could be either a sign for a new counter-jet component that is slowly separating or we start to see the very inner acceleration region of the jet which is not efficiently radiating at radio wavelengths. Further more the image shows transversely resolved jet structures at distances beyond ~0.5pc which facilitate more detailed investigations addressing jet stratification. Analysis of the resolved jet structure shows that the initially wide jet (opening angle ~10deg) collimates within the first parsec into a edge-brightened jet with an opening angle of ~3deg.