Optical Observations Of BL Lacertae In2010-2012

In September, October and November2010and September and October2011, weused Tsinghua-NOAO80cm telescope (TNT) to observe BL Lacertae in BVRI band intotal16nights.Through all these nights the source showed evident variations both intra-night andinter-night. In the observations carried out in2010, BL Lacertae stayed in a relativelypeaceful state. The amplitude of the its variability was about0.1mag every night, andthe averaged magnitude firstly decreased about0.2mag and then increased about0.3mag in these3months. But in the observations in2011BL Lacertae was quite violent,showing variouspatterns of variability. In Sep.202011it had a flare about0.3mag,which is the greatest amplitude in our observations. The inter-night variability is alsoobvious in September2011. In Oct.52011we observed a rapid variability, duringwhich BL Lacertae varied about0.1mag.The variability in the four optical bands is tightly correlated in seen from the lightcurves. In order to make the relationship between different bands clear, we useDCF method to analyze the data in B band and R band in selected3days whenthe time resolution was short. The result suggests that there were no obvioustime lag between these two bands, except for that in Nov.22010the variation ofB band lagged that of R band about1000seconds, namely a hard lag. Thisso-called hard lag is unusual for BL Lacertae, indicating that the acceleratingand cooling of electrons in jet went through a different physical process. Withthe help of formulas for the acceleration and cooling of electrons we predict thatthe magnitude of the radiation zone is about5.2G. The analysis of color indexshows obvious bluer-when-brighter trend in intra-night variation, but the trendwas flat inter-night.Our results indicate that the optical variability properties of BL Lacertaeresemble that of X-ray band of high synchrotron peaked BL Lac objects. Thesimilarities imply common origin of variations, plausibly the most energetictails of the synchrotron emission by the relativistic electrons in the jet.