Variability properties of FR0 radio galaxies in the centimetre range

ABSTRACT Modern radio sky surveys have shown that compact radio sources, so called FR0 radio galaxies, are the majority among the radio-loud active galactic nuclei in the local Universe. The question about the nature of FR0s and their evolutionary status is open. There had been a shortage of the radio continuum measurements of FR0s until recently. We present a study of the variability properties for 32 FR0s in the centimetre range (5–11 GHz) based on the RATAN-600 monitoring in 2020–2025. We show that most of the objects have flat or convex radio spectra with peaks in the MHz–GHz frequency range. Modelling of broad-band radio spectra reveals that a convex spectrum shape can be explained by the synchrotron self-absorption (SSA) processes. The variability of FR0s usually does not exceed 20 per cent on the time-scale of 5 yr, although there are sources with variability up to 50 per cent. We discuss different reasons for the observed FR0 variability. An analysis of the light curves at 5, 8, and 11 GHz allows us to determine characteristic time-scales of 100–900 d and derive constraints on the sizes of emitting regions of about 0.07–0.78 pc. Invoking the available literature data, we investigate the variability properties of some objects on decade time-scales up to 45 yr. We have found objects with long-term trends in the light curves and discuss the GPS source and blazar candidates among the FR0s family.