Radio Variability in Seyfert Nuclei
  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).
