What's So Special About X-ray and Gamma-ray Spectra?

Simulated X-ray spectrum of what could be observed from radio galaxy Hydra A by the future Constellation-X mission.

One reason X-ray spectra are special is that, sometimes, X-ray spectroscopy is simpler to interpret than optical spectroscopy. This is because, at X-ray temperatures, atoms are highly ionized (most of the electrons have been taken away from the atoms), leaving only a few electrons per nucleus. This makes theoretical calculations much easier! Thus it is, in principle, much easier to relate the strengths of X-ray lines to, for example, the abundances of various elements.

A more important reason is that there are many classes of astronomical objects that contain high temperature gases (at millions of degrees K). At these temperatures, more of their energies are radiated as X-rays (both continuum and lines) than at other wavelength ranges, so it makes sense to observe them in the X-rays. Such hot gases can be found, for example, in the corona of the Sun: the observation of the Solar corona is very important because Solar flares and other activities there can affect satellite communication links and the health of astronauts in orbit. Many of the elements that you and I are made of are produced by, or dispersed by, supernova explosions --- and supernova remnants are prominent in the X-ray sky because they are also at X-ray temperatures. Another example is the clusters of galaxies: studies show that the X-ray emitting (and otherwise hard to detect) gas may add up to for far more (in terms of their total mass) than the stars that we can see in the form of these galaxies.

Lines in the optical to X-ray range are produced by atomic (and molecular) processes. In the gamma-ray, though, the lines are produced by different processes. Gamma-ray lines may be produced by nuclear processes --- each species of atomic nuclei has its own own set of characteristic lines, just like atoms do. Nuclear reactions (fission or fusion) leave the nuclei in an unstable state, which then emit gamma-ray lines. In normal stars, all the nuclear fusion takes place at their cores, so the Gamma-rays cannot escape unhindered. However, supernova explosions leave some of these unstable nuclei in the open, and gamma-ray lines from supernova remnants have been detected.

In some X-ray binaries and active galactic nuclei, positrons (anti-particle of the more familiar electron) are produced. A positron and an electron can annihilate each other, creating two gamma-ray photons of 511 keV each.

The technology to observe these gamma-ray lines is not as well developed as at other wavelengths. Still, there have been many important discoveries in the last decade or so, using the Compton Gamma Ray Observatory, among others. The technology is developing rapidly, and we hope to learn a lot about the most violent processes in the Universe using these gamma-ray line in the near future.