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

We report the detection of a simultaneous quasi--periodic oscillation (QPO) in $\gamma$-ray and optical light curves of the blazar 3C 454.3. Periodic flux modulations were detected in both of these wavebands with a dominant period of $\sim 47$ days. This is the first multi-band simultaneous QPO detection in any blazar, and more generally in any AGN, on a timescale of a few tens of days. The QPO lasted for over 450 days (from MJD 56800 to 57250) resulting in over nine observed cycles which is among the highest number of periods ever detected in a blazar light curve. An auto regressive integrated moving average (ARIMA) modelling of the light curve revealed a significant, exponentially decaying, trend in the light curve during the QPO; along with the $47$ days periodicity. We explore several physical models to explain the origin of this transient quasi-periodic modulation 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 to the innermost stable circular orbit of the supermassive black hole, and precessing jets. We conclude that the most likely scenario involves a region of enhanced emission moving helically inside a curved jet. The helical motion gives rise to the QPO and the curvature ($\sim 0.05^{\circ}$ pc$^{-1}$) of the jet is responsible for the observed trend in the light curve.