THE TEV SPECTRUM OF MARKARIAN-501

The energy spectrum of the active galactic nucleus Markarian 501 has b een determined from 0.3 to 10 TeV with the Whipple Observatory Cerenko v Imaging Telescope, by using both small zenith angle and large zenith angle data taken between 1997 February 14 and June 8. The TeV emissio n from Mrk 501 was unprecedentedly high, allowing a statistically accu rate spectrum to be derived. In contrast to previously measured TeV sp ectra, the spectrum over this energy region is not well described by a simple power law. Instead, the spectrum exhibits significant curvatur e and can be well fitted by a parabolic spectrum proportional to E-2.2 2+/-0.04+/-0.05-(0.47 +/- 0.07 log10 E), where the first set of errors is statistical and the second systematic and E is in units of TeV. Si mple power-law fits to the TeV data are also inconsistent with upper l imits from EGRET observations that temporally overlap a subset of the TeV observations. The data show a statistically significant signal abo ve energies of 7 TeV. This energy, combined with variability timescale s, yields a Doppler beaming factor, delta, of at least 1.5.