2012A&A...538A.125Z

We present our optical multiband (B and R) observations of BL Lacertae, which were acquired over 20 nights from May 26 to August 3, 2011, and analyse our data to study the optical emission variability properties of BL Lac. The simultaneous photometric observations were carried out using the 0.8m Tsinghua-NAOC Telescope (TNT) at the Xinglong Observatory of National Astronomical Observatories, Chinese Academy of Sciences (NAOC). TNT is equipped with the Princeton Instrument CCD and standard Johnson-Cousin UBVRI filters. Exposure times were well chosen to make a good compromise between high precision and high temporal density. A multipeak outburst was observed in 2011 observing season. The brightest magnitudes for BL Lac during our observations were B=14.48mag and R=12.89mag at JD=2455709.2810 (May 27). One of the most rapid variations was detected on July 9, when a flux increase of about 0.34mag occurred over 2.8h. In general, both the amplitude of variability and the variation rate are higher at the shorter wavelength. A colour analysis shows that the colour variability processes are apparently complicated. A bluer-when-brighter trend was detected during the outburst. These trends may be the interactive contribution from a ``mildly-chromatic'' component and a ``strongly-chromatic'' one. Meanwhile, intra-night variability in B and R bands was found to be correlated, without any time delay exceeding 2.5-min.