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Astron. Astrophys. 364, 450-454 (2000)

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1. Introduction

The detection of very high energy (VHE) gamma rays from at least 5 high energy cut-off BL Lacs (HBLs) has emphasised the importance of such studies, both to help understand the physics of jets in AGNs and as a probe of the infra-red background. The VHE sources detected to date include Mrk 421 (Punch et al. 1992 , Petry et al. 1996), Mrk 501 (Quinn et al. 1996 , Bradbury et al. 1997), 1ES 2344+514 (Catanese et al. 1998), PKS 2155-304 (Chadwick et al. 1999) and 1ES 1959+650 (Nishiyama et al. 1999). All these sources are characterised by being comparatively close and exhibit a peak in the X-ray region of the spectrum. The VHE emission from these objects has been highly variable, with flaring activity observed on timescales as short as 30 minutes and correlated with the activity at other wavelengths (e.g. Gaidos et al. 1996 , Buckley et al. 1996 , Quinn et al. 1999).

PKS 0548-322 and PKS 2005-489 are the closest Southern hemisphere HBLs. Stecker et al. (1996) have predicted VHE gamma ray fluxes from these objects, based on a simple model relating the X-ray flux to the gamma ray flux and taking into account the expected absorption on the IR background. These predictions confirm that PKS 0548-322 and PKS 2005-489 are strong candidates for detection as VHE emitters with the current generation of telescopes.

PKS 2005-489 was identified as a very bright BL Lac by Wall et al. 1986and was found to be at a distance of [FORMULA] (Falomo et al. 1987). Although it was discovered in the radio-band (and so would formerly have been classified as a radio-selected BL Lac) it is now classified as an HBL or an intermediate object on the basis of its X-ray-to-radio flux ratio (Sambruna et al. 1995 , Perlman et al. 1996). Unusually, it has been detected in the EUV band (Marshall et al. 1995). Although not listed as a 100 MeV gamma ray source in the 2nd or 3rd EGRET catalogues (Thompson et al. 1995 , Hartman et al. 1999) it is seen as a marginal source at energies above 100 MeV (Lin et al. 1996 , Lin et al. 1997 , Lin et al. 1999). It also seen at marginal significance in the GeV EGRET catalogue (Lamb & Macomb 1997).

PKS 0548-322 is another very bright BL Lac at a similar distance of [FORMULA] (Fosbury & Disney 1976). The high X-ray flux and wide band spectral shape indicates that it is a HBL. Observations with OSSE (McNaron-Brown et al. 1995) and EGRET (Thompson et al. 1995 , Hartman et al. 1999) have failed to detect gamma ray emission.

The CANGAROO group have reported limits to VHE gamma ray emission above [FORMULA] TeV for PKS 0548-322 (Roberts et al. 1999) and for PKS 2005-489 (Roberts et al. 1998 , Roberts et al. 1999). We have extended the VHE coverage of these objects down to [FORMULA] GeV using extensive observations made with the University of Durham Mark 6 gamma ray telescope at Narrabri, NSW, Australia from 1996 to 1998. The results of these observations are reported, including those taken during a multiwavelength campaign on PKS 2005-489 (Perlman et al. 1999).

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© European Southern Observatory (ESO) 2000

Online publication: January 29, 2001