A Radio Quasi-periodic Oscillation of 176 day in the Radio-loud
  Narrow-line Seyfert 1 Galaxy J0849+5108
  We analyze the 11-year long-term light curve of the Radio-loud Narrow-line
Seyfert 1 (NLSy1) galaxy J0849+5108 and the nearly simultaneous $\gamma$-ray
data of the source. The data were obtained with the Owens Valley Radio
Observatory 40-m telescope at 15~GHz and with the Large Area Telescope onboard
Fermi Gamma-ray Space Telescope, respectively. A quasi-periodic oscillation
(QPO) signal at a significance of $>$5$\sigma$ is found in the radio light
curve, but no similar modulation is seen in the $\gamma$-ray light curve. The
QPO signal was present for 21 cycles, the longest among the reported radio
QPOs, and likely disappeared after 2019 January 16. Different mechanisms
proposed to explain the observed QPOs in Active Galactic Nuclei (AGN) are
discussed for this QPO case. Either a secular instability in the inner
accretion disk or a helical structure in the jet of J0849+5108 may explain the
radio QPO, while for the latter scenario the jet has to be collimated up to
$\sim$200~pc, similar to that seen in the nearby AGN M87. It will be of
interests to keep monitoring the source at radio frequencies, seeing if similar
QPO signals would appear again or not.
