Multiwavelength study of Cygnus A III. Evidence for relic lobe plasma

TL;DR

This study investigates the particle energy distribution in Cygnus A's cocoon, providing evidence for relic plasma from previous jet activity re-energized by compression, and offers insights into the history of jet activity phases.

Contribution

It presents multiwavelength evidence for relic lobe plasma in Cygnus A, revealing re-energized relic structures and constraining the duration of past jet activity.

Findings

Excess low-frequency emission is from relic plasma, not absorption.

Relic counterlobe and counterjet are re-energized by compression.

The previous jet activity lasted slightly less than the current phase.

Abstract

We study the particle energy distribution in the cocoon surrounding Cygnus A, using radio images between 151 MHz and 15 GHz and a 200 ks Chandra ACIS-I image. We show that the excess low frequency emission in the the lobe further from Earth cannot be explained by absorption or excess adiabatic expansion of the lobe or a combination of both. We show that this excess emission is consistent with emission from a relic counterlobe and a relic counterjet that are being re-energized by compression from the current lobe. We detect hints of a relic hotspot at the end of the relic X-ray jet in the more distant lobe. We do not detect relic emission in the lobe nearer to Earth as expected from light travel-time effects assuming intrinsic symmetry. We determine that the duration of the previous jet activity phase was slightly less than that of the current jet-active phase. Further, we explain some features observed at 5 and 15 GHz as due to the presence of a relic jet.