2022ApJ...926...91F

We observed BL Lac in the B, V, R, and I bands with an 85 cm telescope on nine nights from 2019 September 18 to 2019 December 6. More than 2300 data points were collected. All intraday light curves were examined for variations by using the most reliable power-enhanced F-test and the ANOVA test, and intraday variability was found on five nights. Thanks to our high precision and high temporal resolution data, two key discoveries were made in the following analyses. (1) In addition to the strong bluer-when-brighter behavior on most nights, we observed a color reversal that is rarely found in BL Lac objects. This indicates that there are two different energy distributions of injected electrons on this night. (2) The object traced clockwise loops on the color-magnitude diagrams on one night. These are the first intraday spectral hysteresis loops reported in the optical bands in this object, suggesting interband time lags. We estimated the interband lags by using the ZDCF, ICCF, and JAVELIN, and found the variations in the V and R band lagged that in the B band by about 16 and 18 minutes, respectively. Such optical time lags are expected if the acceleration timescale is much shorter than the cooling timescale.