The Energy Spectrum of the Blazar Markarian 421 above 130 GeV

Markarian 421 (Mrk 421) was the first blazar detected at gamma-ray energiesabove 300 GeV, and it remains one of only twelve TeV blazars detected to date.TeV gamma-ray measurements of its flaring activity and spectral variabilityhave placed constraints on models of the high-energy emission from blazars.However, observations between 50 and 300 GeV are rare, and the high-energy peakof the spectral energy distribution (SED), predicted to be in this range, hasnever been directly detected. We present a detection of Mrk 421 above 100 GeVas made by the Solar Tower Atmospheric Cherenkov Effect Experiment (STACEE)during a multiwavelength campaign in early 2004. STACEE is a ground-basedatmospheric Cherenkov telescope using the wavefront sampling technique todetect gamma rays at lower energies than achieved by most imaging Cherenkovtelescopes. We also outline a method for reconstructing gamma-ray energiesusing a solar heliostat telescope. This technique was applied to the 2004 data,and we present the differential energy spectrum of Mrk 421 above 130 GeV.Assuming a differential photon flux dN/dE proportional to E^-a, we measure aspectral index a = 2.1 +/- 0.2 (statistical) +0.2/-0.1 (systematic). Finally,we discuss the STACEE spectrum in the context of the multiwavelength resultsfrom the same epoch.Comment: 16 pages, 4 figures, submitted to Ap