Rapid Synchrotron Flares from BL Lacertae Detected byASCAandRXTE

We report the variable X-ray emission from BL Lacertae detected in the ASCAToO observation conducted during the EGRET and RXTE pointings, coincident withthe 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 ~3.3 E-11erg/cm2/s. This is more than 3 times higher than the value measured by ASCA in1995. 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 timescale. Together with the requirement of a very steep and varying power lawdominating the soft X-ray band in addition to the hard power law, we suggestthat both the high energy end of the synchrotron spectrum and the hard inverseCompton spectrum were visible in this source during the outburst. We discussthe possible origins of the observed variability time scales, and interpret theshort time scales of the soft X-ray variability as reflecting the size of theemission region.Comment: 20 pages, 4 figures, accepted for publication in the Astrophysical Journa