A Geometric Model to Interpret the γ-Ray Quasiperiodic Oscillation of PG 1553+113

Abstract We propose a jet helical motion model to explain the γ -ray quasiperiodic oscillation of PG 1553+113. In this model, we hypothesize that a relativistic jet with constant angular velocity rotates around the axis, causing the Doppler factor of the jet to vary over time, leading to periodic changes in flux. In addition, we consider that the parameters of the emission field vary slightly during the observation period and can be obtained by directly fitting their multiwavelength spectra. Then, we use the resulting emission field information and the model to reproduce the light curve. Our model successfully explains the 2.18 ± 0.08 yr γ quasiperiodic oscillation of PG 1553+113, which is consistent with previous studies. Most importantly, we notice an upward trend in the light curve, and we suggest that this upward trend is due to long-term variability with a timescale of ∼42 yr.