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 optical and X-ray outburst in 1997. Time-resolved photometry on 1997 August 2 revealed a rapid increase of brightness by about 0.6 mag within 40 min. This large-amplitude, short-time variation is the most active one among known variability of blazars in the optical wavelength. By considering a model based on the "synchrotron self-Compton model" of the relativistic jets of blazars, the size of the variable source should be R similar to 7.2 x 10(14) cm from 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 timescales of the variabilities and down to a possible minimum timescale of around 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 variations of radio-loud AGNs suggests that different mechanisms are responsible for short-term optical variability between BL Lacs and radio-loud AGNs.