Interpretations and Implications of γ -ray Lines from Solar Flares, the Galactic Centre and γ -ray Transients

Observations and theories of astrophysical γ -ray line emission are reviewed and prospects for future observations by the spectroscopy experiments on the planned Gamma-Ray Observatory are discussed. We have discussed in this paper the interpretations and implications of astrophysical γ -ray line observations. Such lines have so far been seen from solar flares, the galactic centre and γ -ray transients. In flares, γ -ray lines are an excellent probe of energetic protons and nuclei. The continuing observations with the γ -ray spectrometer on S.M.M. during the current maximum of solar activity should lead to much new insight into particle acceleration mechanisms and the flare process itself. The 0.511 MeV line from the galactic centre, first observed by balloon-borne detectors, has been confirmed by the HEAO-3 γ -ray spectrometer. The preliminary HEAO-3 data indicate that the line is time-variable, a result whose implication is that the positrons are produced by a discrete source, possibly a massive black hole, within an annihilation region no larger than a light year at the galactic centre. Gamma-ray lines seen in the spectra of γ -ray bursts strongly suggest that neutron stars are the sources of at least some of these bursts. The most commonly observed emission line is in the range from 400 to 450 keV, where it is likely to be gravitationally redshifted positron-electron annihilation radiation. The short duration of the 5 March 1979 burst may reflect the damping of neutron star vibrations by gravitational radiation. No γ -ray lines have yet been seen from processes of nucleosynthesis, but good prospects exist for detecting the lines produced by the decay of ²⁶Al, ⁵⁶Co and ⁴⁴Ti.