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We report on two BeppoSAX observations of BL Lac (2200+420) performedrespectively in June and December 1999, as part of a ToO program to monitorblazars in high states of activity. During both runs the source has beendetected up to 100 keV, but it showed quite different spectra: in June it wasconcave with a very hard component above 5-6 keV (alpha_1 ~ 1.6; alpha_2 ~0.15); in December it was well fitted by a single power law (alpha ~ 0.6).During the first BeppoSAX observation BL Lac showed an astonishing variabilityepisode: the 0.3 - 2 keV flux doubled in ~ 20 minutes, while the flux above 4keV was almost constant. This frequency-dependent event is one of the shortestever recordered for BL Lac objects and places lower limits on the dimension andthe magnetic field of the emitting region and on the energy of the synchrotronradiating electrons. A similar but less extreme behaviour is detected also inoptical light curves, that display non-simultaneous, smaller fluctuations of ~20% in 20 min. We fit the spectral energy distributions with a homogeneous,one-zone model to constrain the emission region in a very simple but effectiveSSC + external Compton scenario, highlightning the importance of the locationof the emitting region with respect to the Broad Line Region and the relativespectral shape dependence. We compare our data with historical radio togamma-ray Spectral Energy Distributions.Comment: 19 pages, 6 figures, accepted by Astronomy & Astrophysic

BL Lacertae: Complex spectral variability and rapid synchrotron flare detected with BeppoSAX