We monitored BL Lacertae simultaneously in the optical B, V, R and I bands for 13 nights during the period 2012-2016. The variations were well correlated in all bands and the source showed significant intraday variability (IDV). We also studied its optical flux and colour behaviour, and searched for inter-band time lags. A strong bluer-when-brighter chromatism was found on the intra-night time-scale. The spectral changes are not sensitive to the host galaxy contribution. Cross-correlation analysis revealed possible time delay of about 10 min between variations in the V and R bands. We interpreted the observed flares in terms of the model consisting of individual synchrotron pulses.
@article{arxiv.1706.07183,
title = {Intraday Optical Variability of BL Lacertae},
author = {Nankun Meng and Jianghua Wu and James R. Webb and Xiaoyuan Zhang and Yan Dai},
journal= {arXiv preprint arXiv:1706.07183},
year = {2017}
}