Investigation of possible systematics in the spectrum of Markarian 501

Energy spectrum measurement of astrophysical sources in the TeV gamma-ray regime is a process which may be prone to systematic effects. Claims of features within the spectrum of any source can be confirmed through various checks independent of the methods used to derive a source spectrum. In this paper we substantiate the curvature in the TeV spectrum of Markarian 501 in 1997 by comparing Whipple gamma-ray data with that of other astrophysical TeV photon sources, the cosmic ray background, and different energy regimes by using different observing elevations.