Very high energy gamma-ray emission from the blazar Markarian 421

Very high energy gamma-ray emission from the BL Lacertae object Markarian 421 has been detected over three observing seasons on 59 nights between 1992 April and 1994 June with the Whipple 10 m imaging Cherenkov telescope. During its initial detection in 1992, its flux above 500 GeV was 1.6 x 10(-11) photons cm(-2) s(-1). Observations in 1993 confirmed this level of emission. For observations made between 1993 December and 1994 April, its intensity was a factor of 2.2 +/- 0.5 lower. Observations on 1994 May 14 and 15 showed an increase over this quiescent level by a factor of similar to 10 (see paper by Kerrick et al.). This strong outburst suggests that four episodes of increased flux measurements on similar timescales in 1992 and 1994 may be attributed to somewhat weaker outbursts. The variability of the TeV gamma-ray emission from Mrk 421 stands in contrast to EGRET observations (see paper by Lin et al.), which show no evidence for variability.