BL Lacertae

4 UBVRI light curves

The UBVRI light curves of BL Lacertae from May 2000 to January 2001 are presented in Fig. 1. It is possible to identify two well-defined phases: the first one starts with a moderate brightness increase, and sees the source in a relatively low brightness state; it ends after one hundred days, when the brightness drops to the minimum value ( $\rm JD=2~451~788$, R=14.50). After that, a rapid brightening of more than $1\rm ~mag$ in a couple of weeks leads to the outburst phase, lasting again about one hundred days. The last available observations witness a final dimming to the pre-outburst levels.

Figure 2: Light curve in the R band during the core WEBT campaign.

In both the two mentioned phases, flux oscillations of several tenths of mag on day time scales are present, and it is interesting to notice that the total variation amplitude is approximately the same (about $0.9\rm ~mag$) in the pre-outburst and in the outburst periods, similar to the results reported for the 1997 outburst of BL Lac (Miller 1999).

One striking feature of Fig. 1 is the exceptional sampling obtained during the core WEBT campaign of July-August 2000, especially in the R band.

The details of the brightness behaviour in the R band during the core WEBT campaign are visible in Fig. 2. The most spectacular variability was detected (with high precision and very good intranight sampling) during the last part of the campaign, plotted in the lower panel of Fig. 2. The boxes indicate periods of particularly interesting variations, which are shown in subsequent figures.

Data taken on August 1-2, August 6-7, and August 8-9, 2000 are plotted in Figs. 3, 4, and 5, respectively. The participating observatories are distinguished by different symbols demonstrating how the observing task moves from east to west as the Earth rotates.

Figure 3:   Light curve of BL Lacertae in the R band on August 1-2, 2000.

Figure 4:   Light curve of BL Lacertae in the R band on August 6-7, 2000.

Figure 5:   Light curve of BL Lacertae in the R band on August 8-9, 2000.

One can notice that, in principle, it is possible to obtain a full-day observing coverage, when both the most eastern and western observatories work, thus succeeding in filling the "Pacific gap''. In practice, this gap is often present in the WEBT light curve, but it is limited to a few hours, which allows one to follow the brightness behaviour almost continuously.

The most impressive variations were detected on August 8-9: a half-mag brightness fall in about $7\rm ~h$, immediately followed by a very steep brightening of $\sim$ $0.4 \rm ~ mag$ in $1.7\rm ~h$.

By looking at the individual flaring episodes in the light curve of Fig. 2, we can see that the most significant rising branches (on July 19, 23, 31, August 1, 3, 6, 7, 9) present brightening rates ranging from 0.11 to $0.22 ~ \rm mag ~ h^{-1}$. Moreover, one can notice that, in general, rising slopes are steeper than dimming ones. However, since the observed events might be the result of the superposition of different "single'' flares, any speculation about the time scales of the underlying brightening and dimming mechanisms is likely not very significant.

BL Lacertae