5@5 -: a 5 GeV energy threshold array of imaging atmospheric Cherenkov telescopes at 5 km altitude

We discuss the concept and the performance of a powerful future ground-based astronomical instrument, 5@5 - a 5 GeV energy threshold stereoscopic array of several large imaging atmospheric Cherenkov telescopes (IACTs) installed at a very high mountain elevation of about 5 km a.s.l. - for the study of the gamma -ray sky at energies from approximately 5 to 100 GeV, where the capabilities of both the current space-based and ground-based gamma -ray projects are quite limited. With its potential to detect the "standard" EGRET gamma -ray sources with spectra extending beyond several GeV in exposure times from 1 to 10(3) s, such a detector may serve as an ideal "gamma-ray timing explorer" for the study of transient non-thermal phenomena like gamma -radiation from AGN jets, synchrotron flares of microquasars, the high energy (GeV) counterparts of gamma ray bursts, etc. 5@5 also would allow detailed gamma -ray spectroscopy of persistent nonthermal sources like pulsars, supernova remnants, plerions, radiogalaxies, and others, with unprecedented for gamma -ray astronomy photon statistics. The existing technological achievements in the design and construction of multi(1000)-pixel, high resolution imagers, as well as of large, 20 m diameter class multi-mirror dishes with rather modest optical requirements, would allow the construction of such a detector in the foreseeable future, although in the longer terms from the point of view of ongoing projects of 100 GeV threshold IACT arrays like HESS which is in the build-up phase. An ideal site for such an instrument could be a high-altitude, 5 km a.s.l. or more, flat area with a linear scale of about 100 m in a very arid mountain region in the Atacama desert of Northern Chile. (C) 2001 Elsevier Science B.V. All rights reserved.