Multiwavelength study of Cygnus A I. Precession and slow jet speeds from radio observations

TL;DR

This study analyzes Cygnus A's radio jets across multiple frequencies, revealing jet precession and slow speeds consistent with independent estimates, and compares precession periods across different AGN types.

Contribution

It provides the first detailed radio-based precession model fit for Cygnus A's jets, demonstrating slow jet speeds and longer precession periods compared to microquasars.

Findings

Jet knots deviate from a straight line, indicating precession.

Precession model fits are plausible with low jet speeds (< 0.5 c).

Precession periods in AGN are longer than in microquasars.

Abstract

We study the jet and counterjet of the powerful classical double FRII radio galaxy Cygnus A as seen in the 5, 8 and 15-GHz radio bands using the highest spatial resolution and signal-to-noise archival data available. We demonstrate that the trace of the radio knots that delineate the jet and counterjet deviates from a straight line and that the inner parts can be satisfactorily fitted with the precession model of Hjellming & Johnston. The parameter values of the precession model fits are all plausible although the jet speed is rather low (< 0.5 c) but, on investigation, found to be consistent with a number of other independent estimates of the jet speed in Cygnus A. We compare the masses and precession periods for sources with known precession and find that for the small number of AGN with precessing jets the precession periods are significantly longer than those for microquasars.