High resolution measurements of the gamma-ray spectrum from the Orion region and a search for such emission lines from other massive star formation regions are essential for a better understanding of these exciting processes. The galactic diffuse gamma-ray emission is the dominant feature of the high-energy gamma-ray sky. The diffuse emission is produced primarily by cosmic-ray electron and proton interactions with the matter (via Bremsstrahlung and nucleon-nucleon interactions) and photons (via inverse Compton interactions) in the interstellar medium. A high-energy gamma-ray telescope with better angular resolution will permit more detailed searches for cosmic-ray gradients including variations in the electron to proton ratio, cosmic ray contrast between the galactic arm/inter-arm regions, and evidence for regions in which the cosmic-ray spectrum differs from the local observed spectrum. Increased sensitivity coupled with improved angular resolution will also allow the flux from fainter gamma-ray point sources to be more accurately separated from the galactic plane diffuse emission. The gamma-ray emission from molecular clouds arises from the same cosmic-ray interactions with matter which produce the general galactic diffuse emission. Molecular clouds provide a means to study these processes and the galactic cosmic rays in localized regions of the galaxy.

Objectives:

• Measure gamma-ray line emission from nearby extra galactic supernovae.
• Confirm the COMPTEL results and better define the energy spectrum to better constrain low-energy cosmic-ray abundances of H, He, and Z>8.
• Imaging spectroscopy of Orion with higher angular resolution mapping to localize the low-energy cosmic-ray sources region.
• Search for nuclear de-excitation lines from other regions of the galaxy to determine the extent of low-energy cosmic-ray acceleration and their role in light element production.

Requirements:

• Flux sensitivity at least a factor of 10 better than INTEGRAL with comparable energy resolution and angular resolution of <1 degree.
  

2.2 THE NATURE OF BLACK HOLES & NEUTRON  STARS

2.2.1 BLACK HOLE SYSTEMS

Less than a decade ago, the only black holes suspected were in massive binaries such as Cyg X-1. The situation has changed dramatically with the discovery of highly-transient compact binary systems with a low-mass stellar companion and a high-mass compact primary (almost certainly a black hole based on the dynamical mass). The estimated total number of these systems in the galaxy may be hundreds or more, and thus they could be the dominant class of X-ray binaries. Black hole systems with a high-mass companion have persistent hard spectra, often extending out to 200 keV, and low-mass companion transient systems have spectra showing broad line emission at around 200 and 400-500 keV, such as that observed in the flaring of Nova Muscae (GRS 1124-684). Spectra of samples of black holes will allow detailed tests of emission models and comparisons with neutron stars. Further comparisons with spectra of AGNs, believed to contain super-massive black holes, could then be made to determine the self-similarity of accretion flows onto black holes over a wide range of mass scales.

2.2.2 ACCRETING NEUTRON STARS: X-RAY BURSTERS AND PULSARS

In low magnetic field (B < 10^8-9 G) neutron star systems, the weak field cannot channel the accretion flow onto the neutron star. When in a state of low accretion, these systems appear to exhibit hard X-ray power law components (hard tails) extending out to ~60-100 keV. Spectral measurements to determine the self-similarity of the photon index (typically 2.5-3) and cutoff energies are of primary interest for understanding the accretion flows onto these systems (versus black holes).