  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.
