Multiband optical variability studies of BL Lacertae

Abstract

We monitored BL Lacertae for 13 nights in optical B, V, R, and I bands during\nOctober and November 2014 including quasi-simultaneous observations in V and R\nbands using two optical telescopes in India. We have studied multi-band optical\nflux variations, colour variation and spectral changes in this blazar. Source\nwas found to be active during the whole monitoring period and showed\nsignificant intraday variability on 3 nights in V and R filters while displayed\nhints of variability on 6 other dates in R passband and on 2 nights in V\nfilter. From the colour-magnitude analysis of the source we found that the\nspectra of the target gets flatter as it becomes brighter on intra-night\ntimescale. Using discrete correlation technique, we found that intraday light\ncurves in both V and R filters are almost consistent and well correlated with\neach other. We also generated spectral energy distribution (SED) of the target\nusing the B, V, R, and I data sets for all 13 nights which could help us\ninvestigate the physical process responsible for the observed variations in BL\nLacertae objects. We also discuss possible physical causes of the observed\nspectral variability.\n