Short-Term Radio Variability and Parsec-Scale Structure in a Gamma-Ray
  Narrow-Line Seyfert 1 Galaxy 1H 0323+342
  We made simultaneous single-dish and very long baseline interferometer (VLBI)
observations of a narrow-line Seyfert 1 galaxy (NLS1) 1H 0323+342, showing
gamma-ray activity revealed by Fermi/LAT observations. We found significant
variation of the total flux density at 8 GHz on the time scale of one month by
the single-dish monitoring. The total flux density varied by 5.5% in 32 days,
which is comparable to the gamma-ray variability time scale, corresponding to
the variability brightness temperature of $7.0 \times 10^{11}$ K. The source
consists of central and southeastern components on the parsec (pc) scale. The
flux of only the central component decreased in the same way as the total flux
density, indicating that the short-term radio variability, and probably the
gamma-ray emitting region, is associated with this component. From the VLBI
observations we obtained the brightness temperatures of greater than $(5.2 \pm
0.3) \times 10^{10}$ K, and derived the equipartition Doppler factor of greater
than 1.7, the variability Doppler factor of 2.2, and the 8 GHz radio power of
$10^{24.6}$ W Hz$^{-1}$. Combining them we conclude that acceleration of radio
jets and creation of high-energy particles are ongoing in the central engine,
and that the apparent very radio-loud feature of the source is due to the
Doppler-boosting effect, resulting in the intrinsic radio loudness to be an
order of magnitude smaller than the observed values. We also conclude that the
pc-scale jet represents recurrent activity from the the spectral fitting and
the estimated kinematic age of pc- and kpc-scale extended components with
different position angle.
