A Quasi-periodic Oscillation in the gamma-ray Emission from the Non-blazar Active Galactic Nucleus PKS 0521-36

TL;DR

This study detects a significant 1.1-year quasi-periodic oscillation in the gamma-ray emission of the non-blazar AGN PKS 0521-36, suggesting gamma-ray outbursts influence the observed periodicity.

Contribution

First detection of a gamma-ray QPO in a mildly beamed jet AGN, using multiple methods to confirm the robustness of the periodic signal.

Findings

Gamma-ray fluxes follow a log-normal distribution.

A 1.1-year QPO is identified with about 5 sigma significance.

Gamma-ray outbursts are linked to the QPO formation.

Abstract

Long-term gamma-ray variability of a non-blazar Active Galactic Nucleus (AGN) PKS 0521-36 is investigated by using Fermi-LAT pass 8 data covering from 2008 August to 2021 March. The results show that the histogram of the gamma-ray fluxes follows a log-normal distribution. Interestingly, in the analysis of about 5.8-year (from MJD 56317 to 58447) LAT data between two outbursts (occurring during 2012 October and 2019 May respectively), a quasi-periodic oscillation (QPO) with a period of about 1.1 years (about 5 sigma of significance) is found in the Lomb-Scargle Periodogram (LSP), the Weighted Wavelet Z-transform (WWZ) and the REDFIT results. This quasi-periodic signal also appears in the results of Gaussian process modeling the light curve. Therefore, the robustness of the QPO is examined by four different methods. This is the first gamma-ray QPO found in a mildly beamed jet. Our results imply that the gamma-ray outbursts play an important role in the formation of the gamma-ray QPO.