Rapid Infrared Variability of Three Radio-loud Narrow-line Seyfert 1
  Galaxies: A View from the Wide-field Infrared Survey Explorer
  Using newly released data from the Wide-field Infrared Survey Explorer, we
report the discovery of rapid infrared variability in three radio-loud
narrow-line Seyfert 1 galaxies (NLS1s) selected from the 23 sources in the
sample of Yuan et al. (2008). J0849+5108 and J0948+0022 clearly show intraday
variability, while J1505+0326 has a longer measurable time scale within 180
days. Their variability amplitudes, corrected for measurement errors, are $\sim
0.1-0.2$ mag. The detection of intraday variability restricts the size of the
infrared-emitting region to $\sim 10^{-3}$ pc, significantly smaller than the
scale of the torus but consistent with the base of a jet. The three variable
sources are exceptionally radio-loud, have the highest radio brightness
temperature among the whole sample, and all show detected $\gamma$-ray emission
in Fermi/LAT observations. Their spectral energy distributions resemble those
of low-energy-peaked blazars, with a synchrotron peak around infrared
wavelengths. This result strongly confirms the view that at least some
radio-loud NLS1s are blazars with a relativistic jet close to our line of
sight. The beamed synchrotron emission from the jet contributes significantly
to and probably dominates the spectra in the infrared and even optical bands.
