ar X iv : a st ro - p h / 02 09 19 4 v 1 1 0 Se p 20 02 STACEE Observations of Markarian 421 During an Extended Gamma - Ray Outburst

The active galaxy Markarian 421 underwent a substantial outburst in early 2001. Between January and May of that year, the STACEE detector was used to observe the source in γ-rays between the energies of 50 and 500 GeV. These observations represent the lowest energy γ-ray detection of this outburst by a ground-based experiment. Here we present results from these observations, which indicate an average integral γ-ray flux of (8.0± 0.7± 1.5)× 10 cms above 140 GeV. We also present a light curve for Markarian 421 as observed by STACEE fromMarch to May, and compare our temporal, as well as spectral, measurements to those of other experiments. Subject headings: galaxies: active—BL Lacertae objects: individual (Markarian 421)—gamma rays: observations Santa Cruz Institute for Particle Physics, University of California, Santa Cruz, CA 95064, USA Enrico Fermi Institute, University of Chicago, 5640 Ellis Ave., Chicago, IL 60637, USA Columbia University & Barnard College, New York, NY 10027, USA Department of Physics, Case Western Reserve University, 10900 Euclid Ave., Cleveland, OH 44106, USA Department of Physics, McGill University, Montreal, Quebec H3A 2T8, Canada Centre for Subatomic Research, University of Alberta, Edmonton, Alberta T6G 2N5, Canada TRIUMF, Vancouver, British Columbia V6T 2A3, Canada Department of Physics & Astronomy, University of California, Los Angeles, CA 90095, USA Present address: Department of Physics, Cornell University, Ithaca, NY 14853 Present address: Laboratoire de Physique Corpusculaire et Cosmologie, Collège de France, F-75231 Paris CEDEX 05, France Present address: Max-Planck-Institut für Kernphysik Postfach 10 39 80, D-69029 Heidelberg, Germany