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A recent multi-year Caltech-NRAO Stripe 82 Survey (CNSS) revealed a group ofobjects that appeared as new radio sources after $>$5--20 years of absence.They are transient phenomena with respect to the Faint Images of the Radio Skyat Twenty Centimeters (FIRST) survey and constitute the first unbiased sampleof renewed radio activity. Here we present the follow-up, radio, optical andX-ray study of them. The group consist of 12 sources, both quasars and galaxieswith wide redshift ($\rm 0.04 < z < 1.7$) and luminosity ($\rm22<log_{10}[L_{1.4GHz}/W~Hz^{-1}]>24.5$) distribution. Their radio propertiesin the first phase of activity, namely the convex spectra and compactmorphology, allow them all to be classified as gigahertz-peaked spectrum (GPS)sources. We conclude that the spectral changes are a consequence of theevolution of newly-born radio jets. Our observations show that over the nextfew years of activity the GPS galaxies keep the convex shape of the spectrum,while GPS quasars rapidly transform into flat-spectrum sources, which mayresult in them not being recognized as young sources. The wide range ofbolometric luminosities, black hole masses and jet powers among the transientsources indicates even greater population diversity in the group of young radioobjects. We also suggest that small changes of the accretion disc luminosity(accretion rate) may be sufficient to ignite low-power radio activity thatevolves on the scale of decades.

Caltech-NRAO Stripe 82 Survey (CNSS) Paper V: AGNs that transitioned to radio-loud state