We present two Beppo SAX observations of BL Lac ( 2200+420 ) as part of a multiwavelength campaign performed in 2000 .
The source was in different states of activity : in June , the X–ray spectrum was faint and hard ( \alpha \sim 0.8 ) , with positive residuals towards low energies .
In October we detected the highest [ 2–10 ] keV flux ever measured for the source .
During this observation , the spectrum was soft ( \alpha \sim 1.56 ) up to 10 keV , while above this energy a hard component was dominating .
The Beppo SAX data are confirmed by simultaneous RXTE short observations .
During the first observation the soft X–ray flux was variable on timescales of a few hours , while the hard X–ray flux was almost constant .
During the second observation , instead , the soft spectrum displayed an erratic behaviour with large variations ( up to factors 3–4 ) on timescales smaller than 2 hrs .
The analysis of the multiwavelength SED of October evidenced an intriguing feature : the optical and X–ray sections of the SED are misaligned , while in the prevailing standard picture , they are both thought to be produced via synchrotron emission .
We suggested four scenarios to account for this discrepancy : a higher than galactic dust–to–gas ratio towards the source , the first detection of bulk Compton emission in the X–ray band , the presence of two synchrotron emitting regions located at different distances from the nucleus , the detection of a Klein–Nishina effect on the synchrotron spectrum .
We evidenced the favorable and critical points of each scenario , but , at present , we can not discriminate between them .