Optical observations of BL Lacertae from 1997 to 1999

¹ Center for Astrophysics, Guangzhou University, Guangzhou 510400, PR China,
² Chinese Academy of Science-Peking University Joint Beijing Astrophysical Center (CAS-PKU.BAC), Beijing, PR China
³ Department of Physics, Yunnan University, Kunming, PR China
⁴ Shanghai Astronomical Observatory, Chinese Academy of Sciences, Shanghai, PR China

Corresponding author: J. H. Fan, jhfan@guangztc.edu.cn

Received: 2 May 2000
Accepted: 9 January 2001

Abstract

We present the optical (V, R, and I) photometry for BL Lacertae, which was observed from 1997 through 1999, with the 1.56-m telescope at the Shanghai astronomical observatory (SHAO). After the 1997 outburst, it dimmed to a low state and then brightened again. During the period JD 2450701 to JD 2450701.5, variations of 0.40 mag, 0.27 mag, and 0.21 mag over a time scale of 100 min were found for V, R, and I bands, suggesting that the variations were decreasing with wavelength. The correlation between V, R, and I is also analyzed using the DCF (Discrete Correlation Function) method. This shows that the variability in the V, R, and I bands are correlated with no time delay longer than 0.2 day.