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

Abstract

<p>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.</p><p>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.</p><p>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.</p><p>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.</p><p>Conclusions. We briefly discuss possible emission models for radio-loud AGN that could explain such QPOs with periods of a few years.</p>