  The High-Altitude Water Cherenkov (HAWC) observatory in central Mexico is
currently the world's only synoptic survey instrument for gamma rays above 1
TeV. Because there is significant interest in covering the full TeV sky with a
survey instrument, we have examined options for a Southern Hemisphere extension
to HAWC. In addition to providing all-sky coverage of TeV sources, a southern
site could complement existing surveys of the densest part of the Galactic
Plane, provide continuous monitoring of Galactic and extragalactic transient
sources in both Hemispheres, and simplify the analysis of spatially extended
signals such as diffuse gamma rays and the TeV cosmic-ray anisotropy. To take
advantage of the air-shower physics and lower the energy threshold of the
experiment as much as possible, a high altitude site above 5000 m a.s.l (vs.
4100 m a.s.l. at the current site in Mexico) has been specified. To facilitate
efficient detector construction at such altitudes, the detector tanks would be
assembled at lower altitude and delivered to the site. An all-digital
communications and data acquisition scheme is proposed. Possible designs
include taking advantage of digital optical module technology from the IceCube
experiment as well as new custom electronics. We discuss the physics potential
of such an experiment, focusing on the energy threshold, angular resolution,
and background suppression capability of the experiment, as well as the
advantages of full-sky coverage above 1 TeV.
