Nature of intranight optical variability of BL Lacertae

Abstract

We present the results of extensive multi-band intra-night optical monitoring\nof BL Lacertae during 2010--2012. BL Lacertae was very active in this period\nand showed intense variability in almost all wavelengths. We extensively\nobserved it for a total for 38 nights; on 26 of them observations were done\nquasi-simultaneously in B, V, R and I bands (totaling 113 light curves), with\nan average sampling interval of around 8 minutes. BL Lacertae showed\nsignificant variations on hour-like timescales in a total of 19 nights in\ndifferent optical bands. We did not find any evidence for periodicities or\ncharacteristic variability time-scales in the light curves.\n The intranight variability amplitude is generally greater at higher\nfrequencies and decreases as the source flux increases.\n We found spectral variations in BL Lacertae in the sense that the optical\nspectrum becomes flatter as the flux increases but in several flaring states\ndeviates from the linear trend suggesting different jet components contributing\nto the emission at different times.\n