Radio-Quiet AGN and the Transient Radio Sky

TL;DR

This study investigates radio variability in Seyfert galaxies, revealing potential intermittent activity and emphasizing the importance of temporal radio observations for understanding faint radio-quiet AGN and their role in cosmic phenomena.

Contribution

It demonstrates that Seyfert galaxies may exhibit nuclear radio variability, especially in compact sources, highlighting the significance of time-domain radio studies for faint AGN.

Findings

Possible correlation between radio variability and absence of large-scale radio emission.

Variability more detectable in compact sources with less jet emission.

Implications for AGN duty cycles and ultra-high energy cosmic rays.

Abstract

8.4-GHZ radio imaging study of an optically selected sample of early type Seyfert galaxies; comparison of images taken at two epochs reveals possible variation in the nuclear radio flux density in five of them over a seven year period. It is shown that there is a possible correlation between the presence of nuclear radio variability and the absence of hundred parsec-scale radio emission, analogous with radio-loud AGN. Our results suggest that all Seyferts may exhibit variation in their nuclear radio flux density at 8.4 GHz, but that variability is more easily recognized in compact sources in which emission from the variable nucleus is not diluted by unresolved, constant flux density radio jet emission within the central 50 pc. Taken in combination with other Seyfert properties, these results suggest a paradigm of intermittent periods of quiescence and nuclear outburst across the Seyfert population and demonstrate the importance of investigating the temporal domain at radio wavelengths, which remains completely unexplored for faint radio-quiet AGN. Discovery of intermittent activity and radio flares has important implications for the AGN duty cycles and the origin of Ultra-High Energy Cosmic Rays. New radio facilities, such as the EVLA/VLBA, eMERLIN, LOFAR and eVLBI on the EVN, will revolutionise the study of radio-quiet AGN; in particular, the combination of increased sensitivity and sampling rate with high-angular resolution and automatic data reduction will open up the transient radio sky and bring major future breakthroughs.