  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.
