4.1.3 NUCLEAR ASTROPHYSICS/MEDIUM ENERGY

The realm of nuclear astrophysics and medium energy gamma rays probes some of the most energetic phenomena in astronomy - the endpoints of stellar evolution in supernovae, neutron stars, and black holes. Measurements in this band can address fundamental questions in astronomy such as star formation, supernova physics, galactic structure, and chemical evolution. The observational challenges of nuclear line astrophysics have been addressed by several missions in the past, beginning with the High Energy Astrophysics Observatory (HEAO) series in the 1970's through the current Compton Gamma Ray Observatory. These missions, along with balloon flight experiments have provided several notable achievements: 1) maps of galactic diffuse ²⁶Al and 0.511 MeV emission with a few degree resolution, 2) study of ⁵⁶Co and ⁵⁷Co lines from the Type II supernova, SN1987A, and interesting limits on ⁵⁶Co from Type Ia SN, 3) detection of cosmic ray induced lines including ¹²C and ¹⁶O emission from the Orion region, and 4) detection of 44Ti from Cas A. Some of these detections are of low statistical significance; better sensitivity is needed to provide diagnostics into the phenomena involved. The planned INTEGRAL mission, which is to be launched in 2001, is expected to provide a significant improvement over existing capabilities by having 2 degree imaging resolution, 2 keV spectral resolution, and a sensitivity to narrow lines of 5x10^-6 photons cm^-2 s^-1 for a 10⁶ s observation of a line of 5 keV width. This is approximately a factor of 10 improvement over OSSE or COMPTEL. At this level, one can begin to make detailed maps of ²⁶Al showing regions of recent star formation and supernovae improvement. A meaningful search for lines from ⁶⁰Fe, and detailed study of both the diffuse and central galactic pair annihilation lines become feasible. However, Integral's sensitivity to diffuse or broadened line emission is degraded. Studying the physics of supernova explosions will, lacking great serendipity, be restricted to Type Ia. Type II and Ib occuring in the Local Group. A Type Ia in Virgo (20 Mpc) would have a flux at peak of about 8x10^-6 photons cm^-2 s^-1. However the lines are broad, about 30 - 40 keV FWHM, so the sensitivity of INTEGRAL is reduced to just detecting the typical event in a 10⁶ s observation. Investigating the physics of SN Ia in Virgo will require greater sensitivity. Development of a nuclear astrophysics mission which applies new technology to improve sensitivity and to address the deficiencies in the current and planned instruments is a priority of gamma-ray astronomy. The goal of this development would be an intermediate mission new start in 2005. The characteristics of such a mission are summarized in Table 4.3. A large field-of-view with good imaging capability is required to provide sensitivity to diffuse emission from the Milky Way as well as to support a high-sensitivity sky survey. An example of a mission concept with these capabilities is a Compton telescope, similar to COMPTEL on CGRO, but using spatially sensitive Germanium detectors. The priority objectives of this mission include:

• Type Ia Supernovae in the Virgo Cluster. With the indicated sensitivity, the mission would detect 2 - 4 Type Ia SN per year from the Virgo cluster at a level of significance considerably better than INTEGRAL. This would permit class studies of 56Ni production and emission line profiles. These measurements provide the best diagnostic of Type Ia structure and burning which have implications for galactic chemical evolution as well as supernova physics.

• Diffuse Galactic Nucleosynthesis. This mission should map the galaxy with good angular resolution in line emissions from ²⁶Al, ⁶⁰Fe, ⁴⁴Ti, ¹²C, ¹⁶O, ⁵⁶Fe and positron annihilation and positronium continuum. These maps will reflect the nucleosynthetic contributions of supernovae, novae and massive stars, discover sites of galactic supernovae, and map interactions of low-energy cosmic rays in the ISM and molecular clouds. The 100-fold improvement in sensitivity and angular resolution over COMPTEL should permit localization of young individual star clusters by the core collapse supernovae remnants they contain.

• Galactic Positron Emission. A large component of the observed galactic 0.511 MeV radiation is likely attributable to galactic Type Ia SN. Thus Ia SNRs could be detected by their residual 0.511 positron annihilation radiation associated with ⁵⁶Co production. Diagnostics of the physical properties can be provided by the line profiles and positronium fraction.

• Galactic Novae. Detection of nuclear emission lines from novae would provide critical tests of the models of novae as thermonuclear runaways on white dwarfs. CNO-rich novae should be detectable in ⁷Be and ²²Na to a distance of 1 kpc and positron annihilation radiation will be detected from all novae within 3 kpc during the first hours of the outburst.
  

Table 4.3. Characteristics of a Nuclear Astrophysics/ Medium Energy Mission using a High Resolution Compton Telescope