Multiband optical variability studies of BL Lacertae

Abstract

We monitored BL Lacertae for 13 nights in optical B, V, R, and I bands during 2014 October and November including quasi-simultaneous observations in V and R bands using two optical telescopes in India. We have studied multiband optical flux variations, colour variation and spectral changes in this blazar. Source was found to be active during the whole monitoring period and showed significant intraday variability on three nights in V and R filters while displayed hints of variability on six other dates in R passband and on two nights in V filter. From the colour–magnitude analysis of the source, we found that the spectra of the target get flatter as it becomes brighter on intranight time-scale. Using discrete correlation technique, we found that intraday light curves in both V and R filters are almost consistent and well correlated with each other. We also generated spectral energy distribution of the target using the B, V, R, and I data sets for all 13 nights which could help us investigate the physical process responsible for the observed variations in BL Lacertae objects. We also discuss possible physical causes of the observed spectral variability.