Astronomy and Astrophysics – Astrophysics
Scientific paper
1999-01-08
Astropart.Phys. 11 (1999) 135-139
Astronomy and Astrophysics
Astrophysics
presented at the VERITAS Workshop on TeV Astrophysics of Extragalactic Sources, eds. M. Catanese, J. Quinn, and T. Weekes, to
Scientific paper
10.1016/S0927-6505(99)00037-7
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.
HEGRA collaboration for the
Konopelko Alex
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