The TeV spectrum of Mkn 501 as measured during the high state in 1997 by the HEGRA stereoscopic system of imaging air Cherenkov telescopes

摘要

The BL Lac object Mkn 501 has shown very high emission in TeV gamma-rays fromMarch to October, 1997. During this period the source was continuouslymonitored with the HEGRA stereoscopic system of 4 imaging air Cherenkovtelescopes for a total exposure time of 110 hours. The unprecedented statisticsof about 38,000 TeV photons, combined with the good energy resolution of ~20 %over the entire energy range and with detailed simulations of the detectorresponse, allowed a determination of the average energy spectrum from 500 GeVto 24 TeV. Although the gamma-ray flux varied strongly with time, the dailyenergy spectra remained rather constant in their shape. Therefore it isjustified to derive a time averaged spectrum and thus to extend, for the firsttime, the spectral measurement well beyond 10 TeV. This TeV spectrum of Mkn 501shows a gradual steepening which could be caused by a number of physicalprocesses, such as a limited energy range of the radiating particles, intrinsicgamma-ray absorption inside the source, the Klein-Nishina effect (in the caseof an inverse Compton origin of the radiation) and finally, an absorption ofthe TeV gamma-rays propagating in the intergalactic medium.