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G. Fossati (UCSD/CASS), M. Capalbi (SAX-SDC, Roma, Italy), A. Celotti (SISSA, Trieste, Italy)
Mkn~421 is the brightest blazar at X--ray and UV wavelengths and the brightest and most highly variable extragalactic TeV source. In the last few years it has been target of several BeppoSAX long lasting campaigns. We summarize the main findings of the 1999 and 2000 campaigns, which spanned several days (4 and 10, in 1999 and 2000 respectively) with continuous coverage. In both campaigns Mkn~421 was in an extremely active state, and in 2000 the flux reached its historical record value of ~q 1.2 \times 10-9 erg cm-2 s-1 in the 2-10 keV band. Moreover, in 2000, simultaneous observations by ground based Cherenkov telescopes reveal that Mkn~421 was in a spectacularly high state in the TeV band, too. The unprecedented quality of these recent observations allows to study in great detail the variability of the extreme tail of the synchrotron component, emitted by the highest energy electrons, which are the most direct probe of the physical conditions in the emitting volume, being most sensitive to the details of the acceleration and cooling mechanisms. This provides us with an important means to put some precise constraints on the energies of the relativistic electrons and on the physical parameters of the plasma within the jet, and for the first time meaningfully constrain the time dependent particle injection/acceleration process.
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