Radio and $\gamma$-ray variability in the BL Lac PKS 0219 -164: Detection of quasi-periodic oscillations in the radio light curve

Abstract

In this work, we explore the long-term variability properties of the blazar PKS 0219-164 in the radio and the $\gamma$-ray regime, utilizing the OVRO 15 GHz and the \fermi/LAT observations from the period 2008--2017. We found that $\gamma$-ray emission is more variable than the radio emission implying that $\gamma$ ray emission possibly originated in more compact regions while the radio emission represented continuum emission from the large scale jets. Also, in $\gamma$-ray the source exhibited spectral variability characterized by the \emph{ softer-when-brighter} trend, a less frequently observed feature in the high energy emission by BL Lacs. In radio, using Lomb-Scargle periodogram and weighted wavelet z-transform, we detected a strong signal of quasi-periodic oscillation (QPO) with a periodicity of 270 $\pm$ 26 days with possible harmonics of 550 $\pm$ 42 and 1150 $\pm$ 157 days periods. At a time when the issue of the detection of QPOs in blazars is still under debate, the observed QPO with high statistical significance ( $\sim$ 97\% -- 99\% global significance over underlying red-noise processes) and persistent over nearly 10 oscillations could make one of the strongest cases for the detection of QPOs in blazar light curves. We discuss various blazar models that might lead to the $\gamma$-ray and radio variability, QPO, and the achromatic behavior seen in the high energy emission from the source.