A recollimation shock 80 mas from the core in the jet of the radio galaxy 3C120: Observational evidence and modeling

TL;DR

This paper presents VLBA observations of the radio galaxy 3C120 revealing a stationary recollimation shock at 80 mas from the core, supported by modeling that explains its brightness and polarization features as a conical shock in the jet.

Contribution

The study provides the first observational evidence and detailed modeling of a recollimation shock in 3C120's jet, linking stationary features to a conical shock structure.

Findings

Identification of a stationary feature C80 as a recollimation shock

Detection of superluminal components downstream of C80

Modeling confirms a conical shock explains observed properties

Abstract

We present Very Long Baseline Array observations of the radio galaxy 3C120 at 5, 8, 12, and 15 GHz designed to study a peculiar stationary jet feature (hereafter C80) located ~80 mas from the core, which was previously shown to display a brightness temperature ~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 (~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 c have been detected downstream from its position, resembling the behavior observed in the HST-1 emission complex in 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 degrees, jet viewing angle \theta=16 degrees, a completely tangled upstream magnetic field, and upstream Lorentz factor \gamma=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 3C120 located at a de-projected distance of ~190 pc downstream from the nucleus.