The phenomenology of BL Lac objects has led to a classification of these sources into two sub-populations: high-frequency BL Lac objects (HBLs) and low-frequency ones (LBLs) (see e.g. Fossati et al. 1998). The HBL objects have synchrotron and Compton peaks at relatively high energies (Ghisellini et al. 1998), and thus are good candidates for TeV emission. With the HEGRA system of IACTs the significant detection (e.g. 3 ) of a flux comparable to the flux from the Crab nebula takes less than 15 minutes for sources close to zenith position. The BL Lac observations presented in this paper are thus sufficient to probe energy fluxes down to the level of 10-12 erg cm-2 s-1 ( GeV).
The two BL Lac objects Mkn 501 and Mkn 421 are well established and well studied TeV sources. Mkn 501 showed during 1997 spectacular outbursts with diurnal flux levels reaching 10 Crab units (e.g., Aharonian et al. 1999a; Samuelson et al. 1998; Djannati-Ataï et al. 1999) and with a spectrum extending up to at least 16 TeV (Aharonian et al. 1999b). For a better understanding of the intrinsic properties of this class of sources and for determining the amount of intergalactic extinction of the TeV photons due to pair production processes with the Cosmic Infrared Background radiation (CIB) photons, the detection of further BL Lac objects is of utmost importance. Thus the search and the detailed study of new extragalactic TeV sources is one of the major objectives of the HEGRA experiment.
The paper is organized as follows: We describe the data samples used in the analysis in Sect. 2 and the method used for computing flux upper limits in Sect. 3. The extinction by the CIB is discussed in Sect. 4, and in Sect. 5 we give the experimental results. In Sects. 6 and 7 we focus on two objects: (1) 1ES 2344+51.4 detected during December 1995 by the WHIPPLE Cherenkov telescope (Catanese et al. 1997), and BL Lacertae which, in July 1997, showed a strong flare detected in gamma-ray by EGRET, in the visible (Bloom et al. 1997; Madejski et al. 1999) and in X-ray wavelengths. A strategy to optimize the search for new extra-galactic sources is briefly outlined in Sect. 8, and the conclusions are given in Sect. 9.
© European Southern Observatory (ESO) 2000
Online publication: January 18, 2000