  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.
