We report on two Beppo SAX 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 different spectra : in June it was concave with a very hard component above 5-6 keV ( \alpha _ { 1 } \sim 1.6 ; \alpha _ { 2 } \sim 0.15 ) ; in December it was well fitted by a single power law ( \alpha \sim 0.6 ) .
During the first Beppo SAX observation BL Lac showed an astonishing variability episode : the 0.3 - 2 keV flux doubled in \sim 20 minutes , while the flux above 4 keV was almost contant .
This frequency–dependent event is one of the shortest ever recordered for BL Lac objects and places lower limits on the dimension and magnetic field 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 of \sim 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 effective 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 \gamma -ray Spectral Energy Distributions .