Radio and γ-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 γ-ray regime, utilizing the OVRO 15 GHz and the Fermi/LAT observations from the period 2008-2017. We found that γ-ray emission is more variable than the radio emission implying that γ ray emission possibly originated in more compact regions while the radio emission represented continuum emission from the large scale jets. Also, in γ-ray the source exhibited spectral variability characterized by the 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 ± 26 days with possible harmonics of 550 ± 42 and 1150 ± 157 days periods. At a time when detections of QPOs in blazars are still under debate, the observed QPO with high statistical significance ( ∼ 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 γ-ray and radio variability, QPO, and the achromatic behavior seen in the high energy emission from the source.

Microvariability in BL Lacertae: “Zooming” into the Innermost Blazar Regions

“…In particular, bluer-when-brighter (alternatively, harder-when-brighter) trend has been frequently observed in the source during intranight observations (see [34,37]). Such an achromatic behavior, both bluer-when-brighter and redder-when-brighter, have been frequently observed in the blazars (for further discussion see [2,19]). Similarly, the optical spectra are found to be of the power-law type consistent with the scenario in which the synchrotron emission contribute by power-law particle injections.…”

“…One of the most important properties that characterize blazars is their variability over a broad range of temporal and spatial frequencies. The variability in general is found to be aperiodic in nature; however, recently, detection of quasi-periodic oscillations in various emission frequencies and timescales have been reported (see [2][3][4]). In particular, low amplitude, rapid variability in the timescales less than a day is widely known as intraday/intranight or microvariability.…”

The Nature of γ-Ray Variability in Blazars

“…In the similar context, highly polarized optical flare discovered by Bhatta et al (2015) might be a signature of dominance of magnetic field near blazar cores, so called magnetically arrested disk (MAD) scenario(see Narayan et al 2003). In observation, detection of QPOs in various kinds of AGNs, including both radio-loud and radio-quiet, on various timescales has been reported in several works (see Bhatta 2019Bhatta , 2018Bhatta , 2017Bhatta et al 2016c, and the reference therein). In addition, QPOs have been observed to naturally develop in numerical studies involving simulations of the parsec scale relativistic jets (e.g.…”

Multi-Wavelength Intra-Day Variability and Quasi-Periodic Oscillation in Blazars

“…In the last decade, several detections of QPOs in AGN on diverse timescales ranging from as short as a few minutes and as long as a few years using γ-ray, X-ray, optical and radio time series data were made (e.g., [122][123][124][125][126][127][128][129][130][131][132][133][134][135][136][137][138][139]; and the references therein). However, only a few claims of QPO detection on IDV timescales using X-ray and optical monitoring data of a few blazars have been reported [123][124][125]127].…”

“…Reference [123] used wavelet analysis, whereas [124] used wavelet plus randomization technique. Weighted wavelet z-transform (WWZ) and the Lomb-Scargle periodogram (LSP) were used by [139]. Multiple analysis techniques, e.g., structure function (SF), wavelet analysis, data folding, PSD and multi-harmonic AoV periodogram (mhAoV), were used by [125].…”