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We report for the first time a gamma-ray and multi-wavelength nearly-periodicoscillation in an active galactic nucleus. Using the Fermi Large Area Telescope(LAT) we have discovered an apparent quasi-periodicity in the gamma-ray flux (E>100 MeV) from the GeV/TeV BL Lac object PG 1553+113. The marginal significanceof the 2.18 +/-0.08 year-period gamma-ray cycle is strengthened by correlatedoscillations observed in radio and optical fluxes, through data collected inthe OVRO, Tuorla, KAIT, and CSS monitoring programs and Swift UVOT. The opticalcycle appearing in ~10 years of data has a similar period, while the 15 GHzoscillation is less regular than seen in the other bands. Further long-termmulti-wavelength monitoring of this blazar may discriminate among the possibleexplanations for this quasi-periodicity.

Multiwavelength Evidence for Quasi-periodic Modulation in the Gamma-ray Blazar PG 1553+113