Multiwavelength Evidence for Quasi-periodic Modulation in the Gamma-ray Blazar PG 1553+113

TL;DR

This study reports the first detection of quasi-periodic oscillations across gamma-ray, radio, and optical wavelengths in the blazar PG 1553+113, suggesting a potential underlying periodic process in its emission.

Contribution

It presents the first evidence of multi-wavelength quasi-periodic oscillations in an active galactic nucleus, combining gamma-ray, radio, and optical data to support the discovery.

Findings

Gamma-ray flux shows a ~2.2-year periodicity.

Correlated optical and radio oscillations observed.

Multi-wavelength data support the quasi-periodicity evidence.

Abstract

We report for the first time a gamma-ray and multi-wavelength nearly-periodic oscillation in an active galactic nucleus. Using the Fermi Large Area Telescope (LAT) we have discovered an apparent quasi-periodicity in the gamma-ray flux (E >100 MeV) from the GeV/TeV BL Lac object PG 1553+113. The marginal significance of the 2.18 +/-0.08 year-period gamma-ray cycle is strengthened by correlated oscillations observed in radio and optical fluxes, through data collected in the OVRO, Tuorla, KAIT, and CSS monitoring programs and Swift UVOT. The optical cycle appearing in ~10 years of data has a similar period, while the 15 GHz oscillation is less regular than seen in the other bands. Further long-term multi-wavelength monitoring of this blazar may discriminate among the possible explanations for this quasi-periodicity.