We report the variable X-ray emission from BL Lacertae detected in the ASCA ToO observation conducted during the EGRET and RXTE pointings, coincident with the 1997 July outburst. The source showed a historically high state of X-ray, optical, and γ-ray emission, with its 2-10 keV flux peaking at ~3.3×10-11 ergs cm-2 s-1. This is more than 3 times higher than the value measured by ASCA in 1995. We detected two rapid flares that occurred only in the soft X-ray band, while the hard X-ray flux also increased, but decayed with a much longer timescale. Together with the requirement of a very steep and varying power law dominating the soft X-ray band in addition to the hard power law, we suggest that both the high-energy end of the synchrotron spectrum and the hard inverse Compton spectrum were visible in this source during the outburst. We discuss the possible origins of the observed variability timescales, and interpret the short timescales of the soft X-ray variability as reflecting the size of the emission region.
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