2025ApJ...995..192Z

We report the discovery of a significant quasiperiodic oscillation (QPO) in the X-ray emission of the TeV blazar 1ES 1959+650 through a detailed timing analysis of long-term Swift X-ray Telescope data. The detected QPO has a period of ∼208 days with a high local confidence level (>5.0σ) and a global confidence level around 3.7σ that persists for over 20 cycles across both high- and low-flux states. The overall variability is composed of this stable QPO superimposed on a slow, long-term trend. Furthermore, we demonstrate that a helical trajectory model for a propagating plasma blob in the jet compellingly reproduces the observed light curve, providing strong support for this physical interpretation.