ar X iv : a st ro - p h / 96 02 06 8 1 4 Fe b 96 VERY HIGH ENERGY GAMMA - RAY EMISSION FROM THE BLAZAR MARKARIAN 421

Very high energy gamma-ray emission from the BL Lac object Markarian 421 has been detected over three observing seasons on 59 nights between April 1992 and June 1994 with the Whipple 10-meter imaging Cherenkov telescope. During its initial detection in 1992, its flux above 500 GeV was 1.6×10−11photons cm−2 s−1. Observations in 1993 confirmed this level of emission. For observations made between December 1993 and April 1994, its intensity was a factor of 2.2±0.5 lower. Observations on 14 and 15 May, 1994 showed an increase over this quiescent level by a factor of ∼10 (Kerrick et al. 1995). This strong outburst suggests that 4 episodes of increased flux measurements on similar time scales in 1992 and 1994 may be attributed to somewhat weaker outbursts. The variability of the TeV gamma-ray emission from Markarian 421 stands in contrast to EGRET observations (Lin et al. 1994) which show no evidence for variability. Subject headings: gamma rays: observations; blazars: individual: Markarian 421. 1University of Michigan, Ann Arbor, MI 48109 2University of Leeds, Leeds LS2 9JT, UK 3Whipple Observatory, Harvard-Smithsonian CfA, Amado, AZ 85645 4Iowa State University, Ames, IA 50011 5St. Patrick’s College, Maynooth, Co. Kildare, Ireland 6University College Dublin, Belfield, Dublin 4, Ireland 7Purdue University, West Lafayette, IN 47907 8Postal Address: Department of Physics and Geology, Northern Kentucky University, Highland Heights, KY 41099 9Postal Address: Instituto de Astronoḿıa y F́ısica del Espacio, C.C. 67, Suc. 28, 1428 Buenos Aires, Argentina