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

We report on two BeppoSAX observations of BL Lac (2200+420) performed respectively in June and December 1999, as part of a ToO program to monitor blazars in high states of activity. During both runs the source has been detected up to 100 keV, but it showed quite dierent spectra: in June it was concave with a very hard component above 5{6 keV (1 1:6; 2 0:15); in December it was well tted by a single power law ( 0:6). During the rst BeppoSAX observation BL Lac showed an astonishing variability episode: the 0.3{2 keV flux doubled in20 min, while the flux above 4 keV was almost constant. This frequency{dependent event is one of the shortest ever recorded for BL Lac objects and places lower limits on the dimension and magnetic eld of the emitting region and on the energy of the synchrotron radiating electrons. A similar but less extreme behaviour is detected also in optical light curves, that display non-simultaneous, smaller fluctuations of20% in 20 min. We t the spectral energy distributions with a homogeneous, one-zone model to constrain the emission region in a very simple but eective SSC + external Compton scenario, highlighting the importance of the location of the emitting region with respect to the Broad Line Region and the relative spectral shape dependence. We compare our data with historical radio to -ray Spectral Energy Distributions.