It is clear that to fully exploit the potential of ground-based -ray astronomy the detection techniques must be improved. This will happen by extending the energy coverage of the technique (with good energy resolution) and by increasing its flux sensitivity (improved angular resolution and increased background rejection). Ideally one would like to do both but in practice there must be trade-offs. Reduced energy threshold can be achieved by the use of larger, but cruder, mirrors, and this approach is currently being exploited using existing arrays of solar heliostats: STACEE (Chantell et al. 1998), CELESTE (Quebert et al. 1995), and Solar-2 (Tümer et al. 1999). A German-Spanish project (MAGIC) (Barrio et al. 1998) to build a 17 m aperture telescope has also been approved. These projects will achieve thresholds as low as 20-30 GeV, where they will effectively fill the current gap in the -ray spectrum from 20 to 200 GeV. Ultimately, this gap will be covered by GLAST (Gehrels & Michelson 1999). Extension to higher energies (> 10 TeV) can be achieved by atmospheric Cerenkov telescopes working at large zenith angles and by particle arrays on very high mountains. An interesting telescope that has just come on line and will complement these techniques is the MILAGRO water Cerenkov detector in New Mexico, which will operate 24 hours a day with a large field of view and will have good sensitivity to -ray bursts and transients (Sinnis et al. 1995).
VERITAS, with seven 10 m telescopes arranged in a hexagonal pattern with 80 m spacing (Fig. 24), will aim for the middle ground between those techniques listed above, with its primary objective being high-sensitivity observations in the 100 GeV-10 TeV range (see the VERITAS Proposal). 1 It will be located in southern Arizona and will be a logical progression from the Whipple Telescope. It is hoped to begin construction in 1999 and to complete the array by 2004.
Figure 24. The proposed arrangement of telescopes in VERITAS. Figure from Weekes et al. (1999).
The German-French-Italian experiment HESS, initially four and eventually perhaps 16 12 m class telescopes in Namibia (Hofmann 1997), and the Japanese Super CANGAROO array, with four 10 m telescopes in Australia (T. Kifune 1999, private communication), will have similar objectives. In each case, the arrays will exploit the high sensitivity of ACITs and the high selectivity of the array approach. The projected sensitivities of MAGIC, HESS, SuperCANGAROO, and VERITAS are somewhat similar, and we refer to them collectively as next-generation gamma-ray telescopes (NGGRTs). The relative flux sensitivities for existing and planned -ray telescopes as a function of energy are shown in Figure 25, where the sensitivities of the wide-field detectors are for 1 year and the atmospheric Cerenkov telescopes are for 50 hours. In all cases, a 5 point-source detection is required.
Figure 25. Comparison of the point source sensitivity of VERITAS to Whipple (Weekes et al. 1989), MAGIC (Barrio et al. 1998), CELESTE/STACEE (Quebert et al. 1995; Chantell et al. 1998), HEGRA (Daum et al. 1997), GLAST (Gehrels & Michelson 1999), EGRET (Thompson et al. 1993), and MILAGRO (Sinnis et al. 1995). The sensitivity of MAGIC is based on the availability of new technologies, e.g., hybrid photomultiplier tubes, not assumed in the other experiments. EGRET, GLAST, and MILAGRO are wide-field instruments and therefore ideally suited for all-sky surveys. The turnup in the VERITAS sensitivity at higher energies is primarily caused by the requirement that the signal contain at least 10 photons.
It is apparent from this figure that, on the low-energy side (< 1 TeV), the NGGRTs will complement the GLAST mission and will overlap with the solar arrays. At the highest energies to which they are sensitive, NGGRTs will overlap with the Tibet Air Shower Array (Amenomori et al. 1997). They will cover the same energy range as MILAGRO but with greater flux sensitivity. The wide-field coverage of MILAGRO will permit the detection of transient sources which, once detected, can be studied in more detail by the northern NGGRTs. These same telescopes will complement the coverage of neutrino sources to be discovered by AMANDA/ICE CUBE (Halzen 1998) at the South Pole. Finally, if the sources of ultrahigh-energy cosmic rays are localized to a few degrees by HiRes (Abu-Zayyad et al. 1997) and Auger (Boratav 1997), the NGGRTs will be powerful instruments for their further localization and identification.
The recent successes in VHE -ray astronomy ensure that in the inevitable interval between the death of EGRET and the launch of the next-generation -ray space telescope, there will be ongoing activity in GeV-TeV -ray astronomy. Observations by GLAST and the NGGRTs in this energy region will make important contributions to our understanding of AGNs, supernova remnants, and pulsar and -ray burst studies. Although the number of TeV sources detected so far is small, the new and varied phenomena observed indicate that VHE -ray astronomy is not merely an extension of MeV-GeV -ray astronomy but is a discipline in its own right. With the advent of new telescopes, the catalog of VHE -ray sources will dramatically expand with detailed time histories and accurate energy spectra available.
We thank F. Aharonian, A. Djannati-Atai, F. Kajino, J. Kataoka, M. Punch, T. Takahashi, D. Thompson, and V. Vassiliev for providing some of the data and figures presented here, and A. Burdett and S. Fegan for reading the manuscript. This research is supported by grants from the US Department of Energy and NASA.
1 T. C. Weekes et al. 1999, VERITAS Proposal (http://veritas.sao.arizona.edu/). Back.