Only two extragalactic sources have been confirmed to emit gamma-rays above 300 GeV: Mkn 421 (Punch et al. 1992, Macomb et al. 1995, Petry et al. 1996) and Mkn 501 (Quinn et al. 1996, Aharonian et al. 1997). In addition, an as yet unconfirmed detection has been made of 1ES 2344+514 (Catanese et al. 1997a). All three have redshifts less than 0.05 and are, based on their values of log (/), classified as X-ray-selected BL Lacs (XBLs). Observations of more than 30 candidates of other source types over a range of redshifts have not yielded any other extragalactic TeV emitters (e.g. Kerrick et al. 1995). From theoretical considerations Stecker et al. (1996) have proposed that low redshift XBLs may be the only extragalactic gamma-ray sources observable at TeV energies.
The CANGAROO data set, containing data collected over more than 5 years, includes observations of four low redshift active galactic nuclei (AGN) of the BL Lacertae class (PKS0521-365, EXO 0423.4-0840, PKS2005-489 and PKS2316-423). In this paper we present the results of searches for TeV emission from these objects. A comparison of on-source and off-source regions of sky is used to search for an excess of gamma-ray-like events from these sources over a typically two-week observing period.
The detection of a large flare from the BL Lac Mkn 421 by the Whipple Collaboration (Gaidos et al. 1996) has encouraged us to analyze the data with particular emphasis on shorter timescale flare searches. The Whipple result has shown that at TeV energies, BL Lacs are capable of extremely energetic flares on timescales of less than 1 day. Preliminary analysis of the Mkn 421 gamma-ray spectrum indicates that photon energies extend beyond 5 TeV (McEnery et al. 1997). Similar strong, short duration flares at TeV energies have also been reported from Mkn 501, showing gamma-ray emission extending to energies of at least 10 TeV (Aharonian et al. 1997, Quinn et al. 1997). The lack of high energy turnover in the observed spectrum implies that the interaction of the gamma-rays with the cosmic infra-red background is at the lower end of expectations (cf Stecker et al. 1992). The results from Mkn 421 and Mkn 501 indicate that the CANGAROO 3.8 m telescope, with a relatively high gamma-ray energy threshold, is capable of detecting such extragalactic sources of TeV gamma-rays.
© European Southern Observatory (ESO) 1998
Online publication: August 6, 1998