Extreme synchrotron blazars : the case of Mkn 501

BeppoSAX observations of Mkn501 in April 1997 (Pian et al. 1998), have revealed an extraordinary X–ray emission from this BL Lac object, during a phase of high activity at TeV energies, as monitored with the Whipple, HEGRA and CAT Cerenkov telescopes. The 0.1–200 keV spectrum was hard, and the X–ray power output peaked at or above 100 keV, 2 or 3 orders of magnitude more than what indicated by previuos observations, while the luminosity increased by at least a factor ∼20. The X–ray spectrum hardens when the source is brighter, but variations seem limited to energies greater than ∼0.5 keV. All these unprecedented spectral information pose severe constraints to all models, and we discuss in particular how the homogenous, one–zone synchrotron self Compton model must be modified to account for the observed properties. Other sources, besides Mkn 501, could undergo similar flares.