Quasi-periodic behaviour in the optical and γ-ray light curves of blazars 3C 66A and B2 1633+38

Abstract

We report on quasi-periodic variability found in two blazars included in the Steward Observatory Blazar Monitoring data sample: the RI_ Lac object 3C 66A and the Flat Spectrum Radio Quasar 132 1633+38. We collect optical photometric mid polarimetric data in V and R bands of these sources from different observatories: St, Petersburg University, Crimean Astrophysical Observatory, WEBT GASP, Catalina Real-Time Transient Survey, Steward Observatory, STELLA Robotic Observatory, and Katzman Automatic Imaging Telescope. In addition, an analysis of the y-ray light curves from Fermi LAI' is included, Three methods are used to search for any periodic behaviour in the data: the Z-trans form Discrete Correlation Function, the Lomb Scargle periodogram and the Weighted Wavelet Z-transform. We find pieces of evidence of possible quasi-periodic variability in the optical photometric data of both sources with periods of 3 yr for 3C 66A and 1.9 yr for B2 1633+38, with significances between 3o- and 50-. Only B2 1633+38 shows evidence of this behaviour in the optical polarized data set at a confidence level of 20-,10-. This is the first reported evidence of quasi periodic behaviour in the optical light curve of B2 1633+38. Also, a hint of quasi-periodic behaviour is found in the y-ray light curve of B2 1633+38 with a confidence level >20-, while no periodicity is observed for 3C 66A in this energy range. We propose different jet emission models that could explain the quasi-periodic variability and the differences found between these two sources.