The TeV spectrum of Markarian 501

The energy spectrum of the active galactic nucleus Markarian 501 has been determined from 0.3 to 10 TeV with the Whipple Observatory Cerenkov Imaging Telescope, by using both small zenith angle and large zenith angle data taken between 1997 February 14 and June 8. The TeV emission from Mrk 501 was unprecedentedly high, allowing a statistically accurate spectrum to be derived. In contrast to previously measured TeV spectra, the spectrum over this energy region is not well described by a simple power law. Instead, the spectrum exhibits significant curvature and can be well fitted by a parabolic spectrum proportional to E-2.22+/-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. Simple power-law fits to the TeV data are also inconsistent with upper limits from EGRET observations that temporally overlap a subset of the TeV observations. The data show a statistically significant signal above energies of 7 TeV. This energy, combined with variability timescales, yields a Doppler beaming factor, delta, of at least 1.5.