Mkn 501, a relatively close BL Lac object (redshift z = 0.034), became a target for observation in the VHE waveband as a result of the unexpected emergence of AGN as a significant sub-class of the high energy -ray sources identified by EGRET onboard the Compton Gamma Ray Observatory (von Montigny et al. 1995 ). Using the atmospheric Cherenkov technique, the Whipple group observed radiation of between 0.5 and 1.5 TeV from the BL Lac object Mkn 421 (Punch et al. 1992 ) and a confirmatory detection by two independent telescopes of the HEGRA Collaboration helped to establish its status as a VHE source (Petry et al. 1996 = Paper I). The Whipple group has since surveyed more than 30 AGN with similar characteristics (Kerrick et al. 1995 ). Of these, one, Mkn 501, has been reported to emit above 300 GeV, at an average flux level of cm-2 s-1 measured between March and July 1995. This was approximately 20 of the average Mkn 421 flux (Quinn et al. 1996 ). Mkn 421 has been detected by EGRET whereas Mkn 501 has not.
The HEGRA Collaboration's imaging air Cherenkov telescopes are part of its cosmic ray detector complex (e.g. Rhode et al. 1996 ) at the Observatorio del Roque de los Muchachos on the Canary Island of La Palma ( N, W, 2200 m a.s.l.). The first telescope, CT1, is described in detail in Mirzoyan et al. (1994 ) and Rauterberg et al. (1995 ). Its energy threshold is approx. 1.5 TeV. It has a 5 m2 reflector as opposed to the 8.5 m2 dishes of the 5 telescope system (CT2-6) now nearing completion (Hermann 1995 ). CT2 and CT3 were targeting other objects whilst CT1 was chosen to monitor Mkn 501.
© European Southern Observatory (ESO) 1997
Online publication: June 30, 1998