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It is perhaps instructive to compare the development of TeV gamma-ray astronomy with that of X-ray astronomy. Table 1 shows roughly similar stages in the development of the respective fields. We think that TeV gamma-ray astronomy is roughly where X-ray astronomy was in the Einstein days, when direct fine imaging became possible for the very first time. Coincidentally, it took both fields about the same amount of time to reach this stage of development, following the detection of the first extra-solar source. A noteworthy difference is that an all-sky survey had been conducted with the Uhuru satellite, before the Einstein satellite was put in orbit, at a flux limit that is several orders of magnitude below the fluxes of the X-ray sources detected at the time. Although a full-sky survey had also been carried out at TeV energies with Milagro (and also Tibet ASgamma), before HESS came online, the flux limit reached is only comparable to the brightest TeV sources known at the time (Atkins et al. 2004).

Table 1. Development of X-ray Astronomy and TeV Gamma ray Astronomy.

Stages X-ray Astronomy TeV Gamma-ray Astronomy

initial activities sounding rocket and balloon experiments Cherenkov and air shower experiments
first detection Sco X-1 Crab Nebula
follow-ups more detections more detections
first survey Uhuru satellite Milagro
direct fine imaging Einstein satellite HESS, VERITAS
further development Ariel 5, ROSAT, ASCA, etc. HAWC? LHAASO? CTA? AGIS?
state-of-the-art survey ROSAT all-sky survey HAWC? LHAASO?
wide- and narrow-field combo RXTE, Swift LHAASO?
state-of-the-art imaging Chandra, XMM-Newton CTA? AGIS?

The Uhuru survey provided much needed guidance to subsequent X-ray missions. It was superseded in the 90s by a much more sensitive survey carried out with the ROSAT satellite. The ROSAT survey saw nearly all classes of astronomical sources, from stars to AGNs to clusters of galaxies, and is truly a key milestone in the development of X-ray astronomy. We believe that a similar comprehensive survey is imperative to the development of TeV gamma-ray astronomy. A glimpse of the importance of such a survey is provided by the HESS survey of the Galactic central region (Aharonian et al. 2005c, 2006c). Though very limited in scope, the HESS survey has led to many of the most exciting recent discoveries in the field. This implies that a full-sky survey with roughly the sensitivity of HESS would be needed to unveil what is below the tip of the iceberg already seen.

Two wide-field surveying experiments have been proposed, based on the water Cherenkov technique that was pioneered by the Milagro collaboration and was proven to be remarkably successful. The High Altitude Water Cherenkov (HAWC) 1 experiment is to be located in Sierra Negra, Mexico, which is about 4100 m above the sea level. In the proposed configuration, HAWC is expected to be 10-15 times more sensitive than Milagro. Being a ground-based experiment, it would be easily expandable (by adding more water tanks) and thus become more sensitive. The Large High Altitude Air Shower Observatory (LHAASO) is still being defined. It is envisioned to spread over an area of one square kilometer and to be located at the Yanbajing (YBJ) cosmic ray observatory in Tibet, China, which is about 4300 m above the sea level. It is worth noting that YBJ now hosts two on-going experiments, Tibet ASgamma and ARGO. Therefore, the excellent infrastructure is already in place for LHAASO. Since the project is still evolving, the ultimate sensitivity of LHAASO is not known at the present time. There are also plans to incorporate Cherenkov telescopes into the observatory, making it a wide-field and narrow-field combo, very much like, e.g., RXTE and Swift, for X-ray astronomy.

Lessons from the success of X-ray astronomy show the importance of parallel development of narrow-field imaging and wide-field surveying experiments. The two are complementary both in technique (see Section 1.1) and in scientific capabilities. Narrow-field imaging experiments enable detailed studies of TeV gamma-ray sources, while wide-field surveying experiments focus mainly on discovering new sources and thus point the way for deep observations with narrow-field experiments. In particular, the latter are most effective in catching transient phenomena that could be associated with supernova or hypernova explosions, merging of neutron stars, nova outburst, blazar outbursts, evaporation of primordial black holes, or processes that have not even been thought of yet. Rapid extragalactic transient gamma-ray signals have been used as probes into some of the most fundamental questions in physics, such as violation of Lorentz invariance. Wide-field surveying experiments can also more easily facilitate multi-wavelength observations, which have proven to be critical to understanding the processes of particle acceleration and radiation production in astronomical environments.

Compared to space-based experiments (such as Fermi), ground-based experiments are easily serviceable and can thus potentially run for a long time. This is important for studies that require high statistical precision (as well as a large sample of sources), such as dark matter search. Ground-based experiments can also be upgraded to improve sensitivity. As long as systematic uncertainties can be controlled at a sufficiently low level, a wide-field experiment like LHAASO has the potential of being an effective pathfinder for the next-generation narrow-field imaging experiments (such as CTA or AGIS). This also represents an excellent opportunity for China to become a major player in the young but exciting field of TeV gamma ray astronomy. To fully expore the promise of wide-field surveying experiments, it would be ideal to have two observatories in the northern and southern hemispheres, respectively, to cover the whole sky. This is certainly another area where international cooperation and collaboration would be critical.

The history of astronomy is full of examples that illustrate how new observational capabilities bring about new discoveries. Even in a branch as mature as radio astronomy, new transient phenomena are still being discovered. For instance, the recent discovery of rotating radio transients (RRATs) shows the presence of an intriguing population of radio pulsars that reveal themselves only in sporadic, ultra-short radio pulses (which last for milliseconds). In optical astronomy, the Sloan Digital Sky Survey (SDSS) has, in many way, changed the way that research is done. There is no pause in the effort. Many new survey experiments are being implemented or proposed, including the Panoramic Survey Telescope and Rapid Response System (Pan-STARRS), the Dark Energy Survey (DES), and the Large Synoptic Survey Telescope (LSST), and they promise to revolutionize the field. At higher energies, the EGRET survey marked the beginning of GeV gamma ray astronomy, which is being further advanced by Fermi. We should not expect TeV gamma ray astronomy to be any different - a strong effort in developing sensitive survey experiments is required to push the field to the next level.


We wish to thank Felix Aharonian and Peter Biermann for providing valuable comments on the manuscript, and Konrad Bernlöhr for providing the figures used in < ahref="Cui1.html#Figure 1">Figure 1. We also acknowledge useful discussions with many participants of the 2008 TeV Particle Astrophysics Workshop in Beijing. This work was partially supported by the US Department of Energy.

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