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   PR # 21128
    Mkn 421 is one of the most interesting BL Lac objects: not only it is
    one of the GeV gamma-ray emitters discoverd by EGRET but also it is  
    the only extra- galactic TeV gamma-ray emitter so far observed by    
    Whipple Telescope. The spectrum in X-ray region shows the spectral   
    breaks which are interpreted as a synchrotron cooling effect.However,
    it is hard to explain both GeV and TeV emission in the standard theor
    etical model(SSC) of the BL Lac objects. We thus propose this observa
    tion as a time critical observation with the EGRET GeV observation on
    May 10-16th, 1994. We will attempt to coordinate with the Whipple tel
    escope for the TeV observation, as well. This concurrent observation 
    will give us crucial information to connect X-ray to GeV/TeV gamma-  
   ray.                          
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