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G. Fossati (CASS/UCSD), A. Celotti (SISSA-ISAS, Trieste, Italy), L. Chiappetti (IFCTR/CNR, Milano, Italy), L. Maraschi (Brera Astron. Obs., Milano, Italy)
Mkn~421 was observed for about two days with BeppoSAX, just prior to the start of a 1 week continuous exposure with ASCA in April 1998, as part of a world-wide multi--\lambda campaign. A pronounced flare event is seen during the first day of observation, simultaneous with a sharp TeV flare detected by the Whipple Observatory telescope, with amplitude of a factor 4 and a halving time of about 1 hour.
The 0.1--2 keV, 4--10 keV and the 2 TeV peaks are simultaneous with each other within one hour but the halving time of the TeV light curve seems definitely shorter. The strong correlation between the TeV and X--ray flares on short timescales, demonstrated here for the first time, supports models in which the TeV radiation arises (via inverse Compton) from the same population of electrons that produce the X--ray flare via synchrotron radiation, and in particular from the same spatial region.
The detailed comparison of 0.1--0.5 keV and 4--5 keV bands light curves suggests that the higher energy band lags the softer one, with a delay of the order of 1000 seconds. This finding is opposite to what is normally found in HBL (High energy peak BL Lacs) X--ray spectra, where usually flares start in hard X--rays and then develop to lower energies.
Moreover, extracting X--ray spectra for \leq 5~ksec intervals, we were able to follow in detail the spectral evolution during the flare, tracking for the first time the shift of the peak of the synchrotron component, moving to higher energy during the rising phase of the flare, and then receding.
The 1998 BeppoSAX data provide then the first evidence that the X--ray and TeV intensities are well correlated on time--scales of hours, and also they would enable us for the first time to give a clue on the acceleration mechanism responsible for the energization of emitting electrons.
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