The TeV Energy Spectrum of Markarian 501 Measured with the Stereoscopic Telescope System of HEGRA during 1998 and 1999

During 1997, the BL Lac object Mkn 501 went into an extraordinary state ofhigh X-ray and TeV gamma-ray activity, lasting more than 6 months. In thispaper we report on the TeV emission characteristics of the source in thesubsequent years of 1998 and 1999 as measured with the Stereoscopic CherenkovTelescope System of HEGRA (La Palma, Canary Islands). Our observations reveal a1998-1999 mean emission level at 1 TeV of 1/3 of the flux of the Crab Nebula, afactor of 10 lower than during the year of 1997. A dataset of 122 observationshours with the HEGRA telescope system makes it possible to assess for the firsttime the Mkn 501 TeV energy spectrum for a mean flux level substantially belowthat of the Crab Nebula with reasonable statistical accuracy. Excluding thedata of a strong flare, we find evidence that the 1998--1999 low-flux spectrumis substantially softer (by 0.44+-0.1(stat) in spectral index) than the 1997time averaged spectrum. The 500 GeV to 10 TeV energy spectrum can well bedescribed by a power law model with exponential cutoff: dN/dE ~ E^(-alpha)exp(-E/E0) with alpha=2.31+-0.22(stat), and E0=5.1 (-2.3+7.8)(stat) TeV. Withinstatistical accuracy, also a pure power law model gives an acceptable fit tothe data: dN/dE ~ E^(-Gamma) with Gamma=2.76+-0.08(stat). After presenting the1998-1999 TeV characteristics of the source we discuss the implications of theresults.