Caltech-NRAO Stripe 82 Survey (CNSS) Paper V: AGNs that transitioned to
  radio-loud state
  A recent multi-year Caltech-NRAO Stripe 82 Survey (CNSS) revealed a group of
objects that appeared as new radio sources after $>$5--20 years of absence.
They are transient phenomena with respect to the Faint Images of the Radio Sky
at Twenty Centimeters (FIRST) survey and constitute the first unbiased sample
of renewed radio activity. Here we present the follow-up, radio, optical and
X-ray study of them. The group consist of 12 sources, both quasars and galaxies
with wide redshift ($\rm 0.04 < z < 1.7$) and luminosity ($\rm
22<log_{10}[L_{1.4GHz}/W~Hz^{-1}]>24.5$) distribution. Their radio properties
in the first phase of activity, namely the convex spectra and compact
morphology, allow them all to be classified as gigahertz-peaked spectrum (GPS)
sources. We conclude that the spectral changes are a consequence of the
evolution of newly-born radio jets. Our observations show that over the next
few years of activity the GPS galaxies keep the convex shape of the spectrum,
while GPS quasars rapidly transform into flat-spectrum sources, which may
result in them not being recognized as young sources. The wide range of
bolometric luminosities, black hole masses and jet powers among the transient
sources indicates even greater population diversity in the group of young radio
objects. We also suggest that small changes of the accretion disc luminosity
(accretion rate) may be sufficient to ignite low-power radio activity that
evolves on the scale of decades.
