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Astron. Astrophys. 327, L5-L8 (1997)

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1. Introduction

Among the TeV cosmic [FORMULA] -ray sources observed by ground-based imaging atmospheric Cherenkov telescopes (IACTs) are two nearby active galactic nuclei (AGNs), Mkn 421 (Punch et al. 1992, Petry et al. 1996) and Mkn 501 (Quinn et al., 1996, Bradbury et al. 1997). In contrast to steady TeV [FORMULA] -ray sources such as the Crab Nebula, the Whipple group (Kerrick et al. 1995a, Gaidos et al. 1996, Buckley et al. 1996) found a dramatic time variability in the radiation from Mkn 421, with characteristic scales as short as one hour. The study of TeV radiation from these and other AGNs is interesting for several reasons. The fast time dependence implies severe limitations on the size of the source, or on the combination of source size and Doppler factor in the case of emission from relativistic jets. TeV [FORMULA] -radiation from such distant objects can furthermore be used to set limits on the diffuse extragalactic background radiation at optical and infrared wavelengths (Biller et al. 1995 and refs given there). TeV [FORMULA] -rays interact with these background radiation fields through pair production, with the cross section peaking near threshold. The observation of a cutoff in the [FORMULA] -ray spectrum at an energy [FORMULA] can be related to the background photon density at the conjugated energy [FORMULA].

In early march 1997, the Whipple group communicated the observation of strong TeV [FORMULA] -ray emission from Mkn 501 at a level well above the flux from the Crab Nebula. Earlier measurements had shown a flux at a level significantly below the Crab flux. Mkn 501 was then detected by the HEGRA IACTs CT1 and CT2, operated independently from each other, and by CAT (Breslin et al., 1997), as well as by the HEGRA IACT system consisting of the four telescopes CT3,4,5,6, used in stereoscopic mode. In this Letter, we report the first results of the analysis of the data obtained with the CT system. Results obtained with CT1 and CT2 will be reported elsewhere.

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© European Southern Observatory (ESO) 1997

Online publication: April 6, 1998