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 ∼ 7.2 × 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
$/-^1\prime\circ$
for timescales of the variabilities and down to a possible minimum timescale of around 10 min. The clear difference between the observed
$\textit{f}\sim ^{10}$
dependence from the canonical
$\textit{f}\sim^{15}-\textit{f}\sim^1\prime^8$
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.