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 \gamma –ray emission , with its 2 – 10 keV flux peaking at \sim 3.3 \times 10 ^ { -11 } erg cm ^ { -2 } s ^ { -1 } .
This is more than 3 times higher than the value measured by ASCA in 1995 .
We detected two rapid flares which occured only in the soft X–ray band , while the hard X–ray flux also increased , but decayed with a much longer time scale .
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 time scales , and interpret the short time scales of the soft X–ray variability as reflecting the size of the emission region .