The BL Lac object Mkn 501 has shown very high emission in TeV gamma-rays from
March to October, 1997. During this period the source was continuously
monitored with the HEGRA stereoscopic system of 4 imaging air Cherenkov
telescopes for a total exposure time of 110 hours. The unprecedented statistics
of about 38,000 TeV photons, combined with the good energy resolution of ~20 %
over the entire energy range and with detailed simulations of the detector
response, allowed a determination of the average energy spectrum from 500 GeV
to 24 TeV. Although the gamma-ray flux varied strongly with time, the daily
energy spectra remained rather constant in their shape. Therefore it is
justified to derive a time averaged spectrum and thus to extend, for the first
time, the spectral measurement well beyond 10 TeV. This TeV spectrum of Mkn 501
shows a gradual steepening which could be caused by a number of physical
processes, such as a limited energy range of the radiating particles, intrinsic
gamma-ray absorption inside the source, the Klein-Nishina effect (in the case
of an inverse Compton origin of the radiation) and finally, an absorption of
the TeV gamma-rays propagating in the intergalactic medium.Comment: presented at the VERITAS Workshop on TeV Astrophysics of
Extragalactic Sources, eds. M. Catanese, J. Quinn, and T. Weekes, to be
published in Astroparticle Physic
