Optical Monitoring and Intraday Variabilities of BL Lacertae

Abstract

Abstract BL Lacertae is a key monitoring target of the 1.26 m National Astronomical Observatory–Guangzhou University Infrared/Optical telescope. Within the monitored duration from 2020 September 4 to 2022 September 28, we report 13,948 observations at the g, r, and i bands (g band: 4498, r band: 4866, i band: 4584). (1) In the monitored duration, this source is located in a very bright and variable state. The maximum variabilities are Δm g = 2.013 ± 0.073 mag at g band, Δm r = 1.900 ± 0.049 mag at r band, and Δm i = 2.279 ± 0.089 mag at i band. (2) Among the gri intraday lightcurves, there are 104 portions of data sets displaying intraday variabilities (IDVs), with the IDV timescales (ΔT) being in the range of 15.84–375.84 minutes and the biggest variable value Δm = 0.430 ± 0.041 mag. (3) The distributions of ΔT show frequency-dependent behavior, and with the frequency increasing, ΔT tend to be shorter. The variable rates ( V = Δ m Δ T ) from the g band are more intense than the values (V r ) from the r band, but are more stable than the values (V i ) from the i band. (4) On three days (2020 September 7, 2020 September 19, and 2022 September 7), we find the intraday periodic oscillations, whose periods are around 150 minutes, 232 minutes, and 150 minutes, respectively, and which might come from the source “flickering.” (5) Based on the distributions between flux densities (F gri ) and spectral indices (α), they show the bluer-when-brighter behaviors and some uneven locations, which should come from the ministructures of the jet, the shock-induced particle acceleration, or magnetic reconnection in the jet.