Long-term Radio Monitoring of PKS 1127–145: Detection and Physical Interpretation of Quasiperiodic Oscillations

Abstract We analyzed over four decades of UMRAO monitoring of PKS 1127–145 at 4.8, 8, and 14.5 GHz to search for quasiperiodic oscillations (QPOs) under red-noise variability. Lomb–Scargle and Weighted Wavelet Z -transform results from 10 day binned light curves were benchmarked against 10 5 Monte Carlo red-noise realizations. We fit autoregressive moving-average (ARIMA) models to 30 day binned light curves to capture stochastic variability, and then assess periodic components by analyzing the periodograms of the model residuals. A ∼1000 day QPO (rest-frame ∼460 days) is locally significant in both the 8 and 14.5 GHz bands, with the strongest global support at 8 GHz; at 4.8 GHz we find evidence for a ∼3300 day component that is weaker and less reliable owing to the limited number of cycles. The confinement of the shorter period to higher frequencies points to a compact, upstream origin. Plausible mechanisms include orbital motion in a supermassive black hole binary, Lense–Thirring precession of a tilted inner flow, and the motion of a relativistic knot along a preexisting helical jet. In the helical case, VLBI kinematics constrain a Doppler factor δ ≈ 21, sufficient to compress an intrinsic ∼26 yr driver into the observed modulation, thereby connecting central-engine dynamics to the radio QPO.