In the technically challenging nuclear line regime, CGRO has uncovered only the tip of the expected MeV line emission science at sensitivities of a few times 10-5 photons cm-2s-1, but already some surprising results have appeared. For example, distribution of radioactive 26A1 mapped by COMPTEL argues for production dominated by massive star death, although surprising excesses in the Vela region are unexplained. Also, the strong COMPTEL detection of 44Ti from Cas A(Figure 3.1.1) implies substantial synthesis of 56Ni. This makes the low luminosity of this Type II supernova event a mystery. Finally, the COMPTEL discovery of what may be broad cosmic-ray induced MeV emission lines from C and O in Orion suggests that the molecular cloud complex is a hotbed of cosmic ray acceleration. The young stellar objects in Orion are thus depositing enormous amounts of energy into shock waves. While the INTEGRAL will provide a significant improvement in sensitivity, efforts to develop new technology in this area will be needed for the field to reach its full potential. We should also note some new research directions in gamma-ray astronomy spurred by CGRO observations. The impact on such problems provides a good measure of the power of future missions. One new area is the association of nonthermal spectra with black hole accretion. In addition to the GeV emission and rapid variability of the blazars, the suprathermal hard X-ray/soft gamma tails in some sources are important diagnostics for disk accretion and processes at the disk inner edge. GRANAT, BATSE, and OSSE measurements above 30 keV show such spectral components in Seyfert AGNs. Intriguingly, when similar features appear in galactic X-ray binaries, dynamical studies have shown the sources to be excellent candidates for black hole accretors. Coupled with this, recent work on accretion disk solutions (e.g., advection dominated disks) suggest that disk inner edge conditions are crucial in producing the optically thin regions that generate such non-thermal spectra. Thus, we can hypothesize that the perfectly absorbing boundary of an accreting black hole is central to the formation of optically thin electron population responsible for the hard X-ray mission and possibly to the acceleration of relativistic jets. We see that the gamma-ray regime provides a unique window on this problem. Our understanding of the AGN phenomenon, likely guided by the "Rosetta Stones"-the galactic black hole candidates-should make a dramatic advance with future hard X-ray to GeV gamma-ray measurements.A second arena where gamma rays draw our attention to the most exotic sources lies in the high-energy emission of gamma-ray bursts. While this emission has been seen in only a handful of events, such emission may be present in most bursts. Conditions needed for the production of this multi-GeV flux, which may dominate the total burst energy, are extreme; we are likely to learn more about burst physics from these radiations than from the more chaotic low-energy emission. This theme of the highest energy providing the sharpest diagnostics is echoed in the study of spin-powered pulsars. While intensively studied in the radio through X-ray bands for over 25 years, the physics of the pulsar magnetosphere is still poorly understood.

CGRO has taught us that much of the well-known pulsar emission is a small fraction of the bolometric luminosity: the power spectrum of their spin-down radiation peaks in the GeV range for some pulsars. Pulsar measurements pose some of the most severe challenges in the gamma-ray range, requiring high sensitivity, precise photon timing, and accurate calibration over the entire keV-GeV range and beyond.A final example illustrates the ability of new gamma-ray data to address fundamenal problems in mainstream astrophysics. The recent detection of several nearby AGNs by ground-based air Cerenkov telescopes with ~300 GeV thresholds illustrates the fact that blazar emission can extendwell beyond the present sensitivity limit of space missions.illustrates the fact that blazar emission can extend well beyond the present sensitivity limit of space missions. Since this gamma-ray flux is attenuated by the ambient cosmic photon fields, we see that measurements of GeV-TeV spectra may fix the soft IR-optical radiation field at high redshift. This new cosmological tool helps to constrain galaxy formation and environments in the early universe. Conversely, the search for absorption affects in GRBs helps to constrain source distances. However, this promise will only be realized via sensitive surveys at the highest energies, coupled with careful ground-based measurements in the TeV range that we can then correlate with our understanding of the early universe. The opportunities for future gamma-ray discoveries are manifold, as are the prospects in many other areas of astrophysics. It is therefore important to note the unique observations at gamma-ray energies.

GRET map of the high energy sky with detected sources
marked.

Figure 1.3 - EGRET map of the high energy sky with detected sources marked. Gamma-ray emitting active galaxics and pulsars can be seen above the diffuse glow of the galaxy.

Above all, gamma-ray astronomy zeros in on some of the most exotic, violent, and fascinating sources: black holes, neutron stars, and explosive sources of nucleosynthesis and particle acceleration. Also, the field is relatively new. With CGRO, gamma-ray measurements have just reached the sensitivity that attract wide theoretical attention and correlative investigations. The rapid development spurred by this synergy should be encouraged over the next decades. Finally, it is a field where new detector development and adoption of technologies used for related terrestrial applications offer the opportunity for dramatic gains. With directed resources for future development and rapid promotion of new technologies to space payloads, certain areas of gamma-ray astrophysics can expect great leaps in sensitivity. These opportunities will be described in the following sections. Thus, with the potential for dramatic new high-energy phenomena strong and the demonstrated ability to probe some of the most exotic objects in the universe, the opportunities for gamma-ray astronomy over the next decade and a half are exciting.


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