Quasi-Periodic Oscillations in the long term radio light curves of the blazar AO 0235+164

TL;DR

This study identifies a significant, persistent quasi-periodic oscillation of approximately 965 days in the radio light curves of the blazar AO 0235+164 over three decades, using multiple analysis methods across three frequencies.

Contribution

It provides robust detection of a long-term quasi-periodic oscillation in a blazar's radio emission, confirmed by multiple analytical techniques and across several frequencies, highlighting its persistent nature.

Findings

Detected a ~965-day QPO in all three radio frequencies.

Identified additional signals at ~1950, ~1350, and ~660 days.

QPO is statistically significant and persistent over three decades.

Abstract

We present time series analyses of three-decade long radio observations of the BL Lacertae object AO 0235+164 made at the University of Michigan Radio Astronomical Observatory operating at three central frequencies of 4.8 GHz, 8.0 GHz and 14.5 GHz. We detected a quasi-periodic oscillation of $\sim$965 days in all three frequency bands in the light curve of the effectively simultaneous observations, along with strong signals at $\sim$1950 d, $\sim$1350 d, and $\sim$660 d. The periodicity is analyzed with three methods: Data Compensated Discrete Fourier Transform, Generalized Lomb-Scargle Periodogram and Weighted Wavelet Z-transform. These methods are chosen as they have different analysis approaches toward robust measurement of claimed periodicities. The QPO at $965\pm 50$ days is found to be significant (at least $3.5\sigma$) and is persistent throughout the observation for all three radio frequencies, and the others, which may be harmonics, are comparably significant in at least the 8.0 GHz and 14.5 GHz bands. We briefly discuss plausible explanations for the origin of such long and persistent periodicity.