Topics in astrophysical x-ray and gamma ray spectroscopy

A number of topics have been investigated relating to astrophysical observations that have already been made or are currently planned of spectral features, mostly emission lines, in the x-ray and gamma ray regions of the electromagnetic spectrum. These topics include: the production of characteristic x-ray and gamma ray lines by non-thermal ions, spectral features induced by processes occurring in strong magnetic fields, and the positron annihilation line at 0.5 MeV. Although astrophysical spectroscopy in this regime is quite young, a number of conclusions can be drawn from existing observations and upper limits, and much can be learned from experiments currently under consideration. First, the rate of x-ray production at 6.8 keV by the 2p to 1s transition in fast hydrogen- and helium-like iron ions was calculated, following both electron capture to excited levels and collisional excitation. A refinement of the OBK approximation was used to obtain an improved charge exchange cross section. This and the corresponding ionization cross section were used to determine the equilibrium charge fractions for iron ions as a function of their energy. The effective x-ray line production cross section was found to be sharply peaked in energy at about 10 MeV/amu. The intense magnetic fields (of order 1012 Gauss) believed to exist at the surfaces of x-ray pulsars could produce observable structure in their spectra. Such fields severely quantize the motion of electrons in the transverse plane, and line emission and absorption can occur accompanying transitions between levels. A previous calculation of the rates for these processes was confirmed; and the cross section for electron-ion Coulomb collisions in strong fields was calculated