Quasi-periodicities at year-like timescales in Blazars

Abstract

We searched for quasi-periodicities on year-like timescales in the light curves of 6 blazars in the optical - near infrared bands and we made a comparison with the high energy emission. We obtained optical/NIR light curves from REM photometry plus archival SMARTS data and we accessed the Fermi light curves for the $\gamma$-ray data. The periodograms often show strong peaks in the optical and gamma-ray bands, which in some cases may be inter-related. The significance of the revealed peaks is then discussed, taking into account that the noise is frequency dependent. Quasi-periodicities on a year-like timescale appear to occur often in blazars. No straightforward model describing these possible periodicities is yet available, but some plausible interpretations for the physical mechanisms causing periodic variabilities of these sources are examined.