The Radio Properties of Radio-Loud Narrow-Line Seyfert 1 Galaxies on
  Parsec Scales
  We present the detection of compact radio structures of fourteen radio-loud
narrow line Seyfert 1 (NLS1) galaxies from Very Long Baseline Array
observations at 5 GHz, which were performed in 2013. While 50\% of the sources
of our sample show a compact core only, the remaining 50\% exhibit a core-jet
structure. The measured brightness temperatures of the cores range from
$10^{8.4}$ to $10^{11.4}$ K with a median value of $10^{10.1}$ K, indicating
that the radio emission is from non-thermal jets, and that, likely, most
sources are not strongly beamed, then implying a lower jet speed in these
radio-loud NLS1 galaxies. In combination with archival data taken at multiple
frequencies, we find that seven sources show flat or even inverted radio
spectra, while steep spectra are revealed in the remaining seven objects.
Although all these sources are very radio-loud with $R > 100$, their jet
properties are diverse, in terms of their milli-arcsecond (mas) scale (pc
scale) morphology and their overall radio spectral shape. The evidence for slow
jet speeds (i.e., less relativistic jets), in combination with the low
kinetic/radio power, may offer an explanation for the compact VLBA radio
structure in most sources. The mildly relativistic jets in these high accretion
rate systems are consistent with a scenario, where jets are accelerated from
the hot corona above the disk by the magnetic field and the radiation force of
the accretion disk. Alternatively, a low jet bulk velocity can be explained by
low spin in the Blandford-Znajek mechanism.
