The 1997 outburst of BL lacertae and detection of a 0.6-mag rapid variation

Photometric observations of BL Lac were carried out during the large optica l and X-ray outburst in 1997. Time-resolved photometry on 1997 August 2 rev ealed a rapid increase of brightness by about 0.6 mag within 40 min. This l arge-amplitude, short-time variation is the most active one among known var iability of blazars in the optical wavelength. By considering a model based on the "synchrotron self-Compton model" of the relativistic jets of blazar s, the size of the variable source should be R similar to 7.2 x 10(14) cm f rom the 40 min variation. The power-density spectrum of the light curve on the night yielded a dependence of the power to the frequency f(-1.0) for ti mescales of the variabilities and down to a possible minimum timescale of a round 10 min. The clear difference between the observed f(-1.0) dependence from the canonical f(-1.5)-f(-1.8) power indices observed in X-ray variatio ns of radio-loud AGNs suggests that different mechanisms are responsible fo r short-term optical variability between BL Lacs and radio-loud AGNs.