Extended X-ray emission in PKS 1718$-$649

TL;DR

This study uses combined Chandra and XMM-Newton data to analyze extended X-ray emission around the young AGN PKS 1718-649, revealing a hot, collisionally ionized medium likely influenced by supernovae rather than AGN feedback.

Contribution

First detailed analysis of extended X-ray emission in a GPS/CSO source using combined archival X-ray data, identifying the nature of the emitting gas.

Findings

Extended soft X-ray emission is confirmed around PKS 1718-649.

The emission originates from a hot, collisionally ionized medium.

Supernovae likely produce the observed X-ray emission, not AGN feedback.

Abstract

PKS 1718$-$649 is one of the closest and most comprehensively studied candidates of a young active galactic nucleus (AGN) that is still embedded in its optical host galaxy. The compact radio structure, with a maximal extent of a few parsecs, makes it a member of the group of compact symmetric objects (CSO). Its environment imposes a turnover of the radio synchrotron spectrum towards lower frequencies, also classifying PKS 1718$-$649 as gigahertz-peaked radio spectrum (GPS) source. Its close proximity has allowed the first detection of extended X-ray emission in a GPS/CSO source with Chandra that is for the most part unrelated to nuclear feedback. However, not much is known about the nature of this emission. By co-adding all archival Chandra data and complementing these datasets with the large effective area of XMM-Newton, we are able to study the detailed physics of the environment of PKS 1718$-$649. Not only can we confirm that the bulk of the $\lesssim$kiloparsec-scale environment emits in the soft X-rays, but we also identify the emitting gas to form a hot, collisionally ionized medium. While the feedback of the central AGN still seems to be constrained to the inner few parsecs, we argue that supernovae are capable of producing the observed large-scale X-ray emission at a rate inferred from its estimated star formation rate.