Between March 16 , 1997 and April 14 , 1997 , a high flux level of TeV \gamma -rays was observed from Mkn 501 , using the HEGRA stereoscopic system of four imaging Cherenkov telescopes . The flux level varied during this period from about one half up to six times the flux observed from the Crab Nebula . Changes of the detection rate by a factor of up to 4 within 1 day have been observed . The measured differential energy spectrum of the radiation follows a power law from 1 TeV to 10 TeV . The differential spectral index of 2.47 \pm 0.07 \pm 0.25 is close to that of the Crab Nebula of 2.66 \pm 0.12 \pm 0.25 .