Analysis of Long-period Optical Variation and Study on Color Index Variation about Optical Band in FSRQ 0208-512

The observational data of optical B, V, and R bands of the Flat Spectral Radio Quasar (FRSQ) 0208-512 are collected from the Small and Moderate Aperture Research Telescope System (SMARTS) database. The data span nearly a decade from February 2008 to August 2017, which includes 627, 639, and 634 sets of data in B, R, and V bands, respectively. The datasets are fully analyzed by the Weighted Wavelet Z Transform (WWZ) and the Monte-Carlo Simulation LombScargle Periodogram (LSP) method, the conclusions are as follows: (1) a 396 d period is obtained in all three optical bands, which is about 1.08 yr; (2) a positive relationship is found between the magnitude of B band (B) and the optical band color index (B - V), which means the smaller the magnitude B the smaller the color index (B - V), this is the so called Bluer-When-Brighter; (3) using the 1.08 yr long-period obtained from the optical bands, the central black hole mass M is derived as 0.12 × 10₆ M ⊙ and the corresponding radius of the radiation zone R_M=1.052×10⁸ km, here M ⊙ is the solar mass.