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We report the detection ($>4\sigma$) of a Quasi-Periodic Oscillation (QPO) inthe $\gamma$-ray light curve of 3C 454.3 along with a simultaneous marginal QPOdetection ($>2.4\sigma$) in the optical light curves. Periodic flux modulationswere detected in both of these wavebands with a dominant period of $\sim 47$days. The $\gamma$-ray QPO lasted for over 450 days (from MJD 56800 to 57250)resulting in over nine observed cycles which is among the highest number ofperiods ever detected in a blazar light curve. The optical light curve was notwell sampled for almost half of the $\gamma$-ray QPO span due to the daytimetransit of the source, which could explain the lower significance of theoptical QPO. Autoregressive Integrated Moving Average (ARIMA) modelling of thelight curve revealed a significant, exponentially decaying, trend in the lightcurve during the QPO, along with the $47$ days periodicity. We explore severalphysical models to explain the origin of this transient quasi-periodicmodulation and the overall trend in the observed flux with a month-like period.These scenarios include a binary black hole system, a hotspot orbiting close tothe innermost stable circular orbit of the supermassive black hole, andprecessing jets. We conclude that the most likely scenario involves a region ofenhanced emission moving helically inside a curved jet. The helical motiongives rise to the QPO and the curvature ($\sim 0.05^{\circ}$ pc$^{-1}$) of thejet is responsible for the observed trend in the light curve.

Multiwaveband Quasi--periodic Oscillation in the Blazar 3C 454.3