Many review articles and topics related to gamma-ray astronomy start with a statement that the 7-ray domain of cosmic radiation is the last of the electromagnetic windows to be opened. This concerns not only the first papers written several decades ago by the pioneers of gamma-ray astronomy, but also the recent assessments of the status of the field. Actually, a gamma-ray astronomer may argue that this window is already opened, at least in the sense that we already have a map of the sky in 7-rays (see Fig. 1.1). Gamma-ray astronomy has indeed entered the main stream of modern astrophysics with more than 300 reported sources, and reliable detection techniques with a potential for further significant improvement. Moreover, many "hot" topics of modern astronomy like the physics and astrophysics of relativistic jets in Active Galactic Nuclei (AGN) essentially rely on 7-ray observations in the MeV (106 eV), GeV (109 eV), and TeV (1012 eV) energy regions. On the other hand, it is difficult to object to the most critical representatives of the advanced areas of astronomy who argue that the performance of current gamma-ray detectors (in particular the flux sensitivities and angular resolution) needs to be improved significantly in order to match the performance of Radio, Optical and X-ray telescopes. The fact that a major fraction of more than 270 sources detected by the Energetic Gamma Ray Experiment Telescope (EGRET) aboard the Comp-ton Gamma Ray Observatory (GRO) remains unidentified, supports, to a large extent, such a critical view. The ratio of identified-to-unidentified sources is significantly better in the TeV region – only one unidentified source from more than 10 reported objects (see Fig. 1.1). However, in the TeV regime presently we deal with an astronomy with a very limited number of detected sources. Thus, one may conclude that the Y-ray sky remains a largely unexplored frontier representing one of the last energy bands of the electromagnetic spectrum to be explored with detectors of an adequate sensitivity.
With the new generation of space-based and ground-based detectors, the energy flux sensitivity will be significantly improved approaching to the level between 10-13 and 10-12 erg/cm2s over a broad energy range from 100 MeV to 10 TeV (see Fig. 1.2). For example, the flux sensitivity at GeV energies will be improved (for point-like sources) by the Gamma-ray Large Area Space Telescope (GLAST) by two orders of magnitude. It is expected that with GLAST we will enter an era of "gamma-ray astronomy with thousands of sources". Dramatic improvements are expected also in the TeV regime. There is a confidence that the new generation of stereoscopic arrays of Imaging Atmospheric Cherenkov Telescopes (IACTs) with energy threshold around 100 GeV will bring many important results and discoveries relevant to various aspects of high energy astrophysics and cosmology.
Fig. 1.1 The reported MeV/GeV (EGRET) and TeV gamma-ray sources. Note that the presentation of the locations of EGRET sources by symbols with variable size is chosen to illustrate the level of Y-ray fluxes detected from individual sources (Hartman et al., 1999). The locations of TeV sources are shown by larger symbols in order to distinguish them from EGRET sources, but they do not correlate with the reported flux or source angular size.
Another high priority objective of future instrumental developments will be an attempt of exploration of the energy interval between 10 and 100 GeV. The interest to this relatively narrow energy band is motivated not only by the natural desire to enter a new domain which remains a terra incognita, but also because it provides a bridge between the high and very high astronomies, and thus may allow key inspections of the current concepts concerning both the GeV and TeV regimes. Moreover, this energy region is crucial for proper understanding of a number of astrophysical and cosmological phenomena related to the physics and astrophysics of AGN and Gamma-Ray Bursts (GRBs).
Fig. 1.2 Energy flux sensitivities of the future low-energy (INTEGRAL, MEGA) and High Energy (GLAST) space-based detectors shown together with flux sensitivities of the current (Whipple, HEGRA), upcoming "100 GeV" threshold (e.g. H.E.S.S.) and future "sub-10 GeV" threshold (e.g. 5@5) arrays of Imaging Atmospheric Cherenkov Telescopes. For comparison, the predicted synchrotron (S) and inverse Compton (IC) fluxes, as well as the reported 7-ray fluxes.
In spite of lack of information about 7-ray sources in this energy region, the extension of a hard diffuse extragalactic 7-radiation detected by EGRET up to 100 GeV (see Fig. 1.3), which represents the 7-ray emissivity of the entire Universe, is a clear indication of the ongoing nonthermal activity in the Universe with significant release of power in this energy band. To a large extent, this general statement does not depend on the specific origin of this radiation, in particular on the question whether it is mainly contributed by discrete unresolved sources or it is a product of truly diffuse processes that take place in the intergalactic medium. Different aspects of gamma-ray astronomy in this intriguing energy interval will be be effectively covered by GLAST and, hopefully, also by arrays of low energy-threshold Cherenkov telescopes (see Fig. 1.2).