Very Long Baseline Array Imaging of Parsec-scale Jet Structures in
  Radio-loud Narrow-line Seyfert 1 Galaxies
  We conducted very long baseline interferometry (VLBI) observations of five
radio-loud narrow-line Seyfert 1 (NLS1) galaxies in milliarcsecond resolutions
at 1.7 GHz (18 cm) using the Very Long Baseline Array (VLBA). Significant
parsec-scale structures were revealed for three out of the five sources with
high brightness temperature by direct imaging; this is convincing evidence for
nonthermal jets. FBQS J1644+2619 with an inverted spectrum showed a prominent
one-sided linear structure, indicating Doppler beaming with an intrinsic jet
speed of >0.74c. FBQS J1629+4007, also with an inverted spectrum, showed rapid
flux variability, indicating Doppler beaming with an intrinsic jet speed of
>0.88c. Thus, we found convincing evidence that these two NLS1s can generate at
least mildly or highly relativistic jets, which may make them apparently radio
loud even if they are intrinsically radio quiet. On the other hand, the other
three NLS1s had steep spectra and two of them showed significantly diffuse
pc-scale structures, which were unlikely to be strongly beamed. Thus, some
NLS1s have the ability to generate jets strong enough to make them
intrinsically radio loud without Doppler beaming. NLS1s as a class show a
number of extreme properties and radio-loud ones are very rare. We build on
these radio results to understand that the central engines of radio-loud NLS1s
are essentially the same as that of other radio-loud AGNs in terms of the
formation of nonthermal jets.
