Analysis of Quasi-periodic Variability in the Optical Light Curve of Blazar 3C 66A

We searched the quasi-periodic variability in the light curve of blazar 3C 66A in the optical V band. The data used in this paper are from Shanghai Astronomical Observatory (ShAO), AAVSO (The American Association of Variable Star Observers), and Steward Observatory respectively, with a total coverage of $\sim$18 years (from 2003 to 2021). Two methods were employed to search for the possible quasi-periodic variability in the resulting optical light curve of the source. The Jurkevich method yielded two possible periods of (850 $\pm$ 90) d ($\sim$2.3 yr) and (1150 $\pm$ 140) d ($\sim$3.2 yr). While the Lomb-Scargle Periodogram, with the red noise considered, yielded two possible periods of (869 $\pm$ 70) d and (1111 $\pm$ 90) d, which are consistent with the results of the Jurkevich method. The significance level of the two periods calculated using the PSRESP (power spectra response) method is $>$ 99% and $>$ 95% for the two possible periods, respectively. We compared our results with previous researches, and found that the period around 2.3 yr is stable in the historical light curve of 3C 66A, while the later one is more likely a transient phenomenon which is only present in the high state of the source.