Detection of quasi-periodic oscillations in the 37 GHz radio light curve of the blazar Ton 599 during 1990─2020

Abstract

Context. Blazars are a subclass of radio-loud active galactic nuclei (AGN) that display strong multi-wavelength variability on diverse timescales ranging from years down to minutes. In the last 1.5 decades, there have been occasional detections of quasi-periodic oscillations in several blazars in their time series data. Aims. We searched for quasi-periodic oscillations (QPOs) in the 37 GHz radio band light curve of the flat-spectrum radio quasar Ton 599 made at the RT-22 radio telescope in Simeiz, Crimea, from 1990 to 2020. We also searched for QPOs in the available gamma-ray and optical data during the time span of these radio observations. Methods. To identify and quantify the QPO nature of this radio light curve of Ton 599, we used the Lomb─Scargle periodogram (LSP), REDFIT, and weighted wavelet Z-transform (WWZ) analyses. We performed LSP analyses of the gamma-ray and optical data. Results. We report the detection of a likely QPO of about 2.4 years in a portion of the 37 GHz radio light curve of Ton 599. No QPO signatures of similar timescales were found in either the γ-ray or optical (R-band) wavebands. Conclusions. We briefly discuss possible emission models for radio-loud AGN that could explain such QPOs with periods of a few years.