X-Ray Atomic Data
In astrophysics most of the X-ray emission is produced either by one of two processes. Stellar coronae, supernova remnants, galaxies and clusters of galaxies show emission features primarily produced by collisions between electrons and ions. Black holes and neutron stars produce strong high energy continuum emission by unknown processes; this emission in turn photoionizes the surrounding environment. We look through this environment and may see emission or absorption features implanted on the continuum.
Spectral models which include large numbers (thousands to even millions) of spectral features are used to interpret the X-ray spectra. Properties of the spectral features are compiled into large atomic databases to support this modeling effort. The databases require accurate and complete data, and are routinely updated with improvements from atomic theory. High energy astrophysics thus relies on the microphysics of ions and electrons to understand the macrophysics of extreme environments.
The Bohr model of the Hydrogen atom assumes that a single electron travels around a positively charged nucleus. Like the planets in the solar system, the orbit is circular but the forces are electrostatic rather than gravitational. From quantum mechanics, only certain orbits are allowed. The electron can jump between these fixed orbits by absorbing or emitting energy. Jumps to the lowest orbit (or energy level) produce the famous Lyman series of Hydrogen, with the one depicted in the figure known as "Lyman alpha," with a wavelength of 1216 Angstroms.
Elements other than Hydrogen exhibit "Hydrogen-like" spectra when they only have one electron. In the Bohr model, the wavelength of light is proportional to (1/Z)2, where Z is the charge of the nucleus. For example, when Neon has only one electron, it shows a line at a wavelength of 12.16 Angstroms (actually it's at 12.13 Angstroms, because there are corrections to the simple model, but the simple calculation works pretty well!). This is very close to 1.0 keV, near the peak sensitivity of the Chandra High Energy Transmission Grating. Neon with 2 electrons has a spectrum similar to Helium, and so on. Highly stripped ions of Oxygen, Neon, Magnesium, Silicon, Sulfur and Iron are routinely measured with Chandra.
Adam Foster, Nancy Brickhouse, Randall Smith