The Energy Spectrum of the Blazar Markarian 421 Above 130 GeV

J. E. Carson, J. Kildea, R. A. Ong, J. Ball, D. A. Bramel, C. E. Covault, D. Driscoll, P. Fortin, D. M. Gingrich, D. S. Hanna, T. Lindner, C. Mueller, A. Jarvis, R. Mukherjee, K. Ragan, R. A. Scalzo, D. A. Williams, J. Zweerink

Published 2006-12-19Version 1

Markarian 421 (Mrk 421) was the first blazar detected at gamma-ray energies above 300 GeV, and it remains one of only twelve TeV blazars detected to date. TeV gamma-ray measurements of its flaring activity and spectral variability have placed constraints on models of the high-energy emission from blazars. However, observations between 50 and 300 GeV are rare, and the high-energy peak of the spectral energy distribution (SED), predicted to be in this range, has never been directly detected. We present a detection of Mrk 421 above 100 GeV as made by the Solar Tower Atmospheric Cherenkov Effect Experiment (STACEE) during a multiwavelength campaign in early 2004. STACEE is a ground-based atmospheric Cherenkov telescope using the wavefront sampling technique to detect gamma rays at lower energies than achieved by most imaging Cherenkov telescopes. We also outline a method for reconstructing gamma-ray energies using 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 a spectral index a = 2.1 +/- 0.2 (statistical) +0.2/-0.1 (systematic). Finally, we discuss the STACEE spectrum in the context of the multiwavelength results from the same epoch.