Radio Variability in Seyfert Nuclei

TL;DR

This study investigates radio flux variability in Seyfert galaxy nuclei at 8.4 GHz, suggesting all Seyferts may vary but are more detectable when compact, and proposing that jets can be temporarily relativistic during flares.

Contribution

It provides the first evidence that Seyfert nuclei exhibit radio variability and links this variability to jet activity and episodic outbursts, expanding understanding of AGN behavior.

Findings

Possible radio variability in 5 of 11 Seyferts over 7 years.

No clear correlation between variability and Seyfert type or luminosity.

Variability is more detectable in compact sources without extended radio emission.

Abstract

Comparison of 8.4-GHz radio images of a sample of 11 early-type Seyfert galaxies with previous observations reveals possible variation in the nuclear radio flux density in 5 of them over a 7-yr period. We find no correlation between radio variability and nuclear radio luminosity or Seyfert nuclear type, although the sample is small and dominated by type 2 Seyferts. Instead, a possible correlation between the presence of nuclear radio variability and the absence of ~100-pc-scale radio emission is seen. NGC2110 is the only source with significant extended radio structure and strong nuclear variability (>38% nuclear decline over seven years). 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 recognised 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. If flares in radio light curves correspond to ejection of new relativistic components or emergence of shocks in the underlying flow, we suggest that radio jets may be intrinsically non-relativistic during quiescence, but that Seyferts, as black-hole driven AGN, have the capacity to accelerate relativistic jets during radio flares. Taken together with the increased detection rate of flat spectrum radio nuclei in Seyferts imaged at VLBI resolutions and the detection of variable water megamaser emission, our results support the paradigm of intermittent periods of quiescence and nuclear outburst across the Seyfert population. (Abridged).