IN THIS X-RAY IMAGE taken by the Chandra X-Ray Observatory, elliptical galaxy Centaurus A, also known as NGC 5128, shows a bright central source called an active galactic nucleus, a jet of gas and diffuse points that glow in the heat of several-million-degree gas. The unprecedented resolution of spectrographs on Chandra and the XMM-Newton satellite are now allowing scientists to tease out important details about the composition of such objects. Image: NASA/SAO/R. KRAFT ET AL.
Astronomy is not usually associated with high-precision measurements. Consider: this is a field in which results given to within a factor of two are still received without scorn. No one in the field seemed particularly fazed, for example, when it was erroneously announced, not long ago, that some objects (called globular clusters, groups of tightly packed stars) were measured to be older than the universe itself.
But all that is changing fast. Across most of the electromagnetic spectrum, extending from the longest radio waves to the shortest cosmic rays, astronomers now have a sharper view of objects of study. This is thanks to newer instruments that provide precise spectroscopy, which offers resolutions and light-collecting capabilities many times more sensitive than those from just a few years ago (see sidebar on spectrographs). Spectroscopy is the science of measuring line emissions from elements and using these measurements to get clues on the state (temperature, density, ionization) of the atoms.
The recent data from the new and better instruments have invalidated some old theories, allowed new ones to flourish and uncovered unexpected results. In short, they have triggered a discovery fever that is changing astronomy. A quick look at an astronomical database called the NASA Astrophysics Data System reveals that the number of papers linked to high-resolution spectral analysis submitted in the first two months of 2002 was greater than that from all the recorded years before 1996 combined.
Among the recent findings is the discovery of planets in other stellar systems (see sidebar). Perhaps most dramatic, however, has been a series of revelations in x-ray astronomy, which is marking its 40th anniversary in 2002. When two x-ray satellites, the NASA Chandra X-Ray Observatory and the European Space Agency's XMM (X-Ray Multi-Mirror)-Newton, both launched two years ago, x-ray astronomy leaped ahead.
The study of the universe in x-rays, which reveal details of some of the most high-energy events in the cosmos, is a relatively new addition to astronomy. That's because every x-ray detector has to be space- or airborne (see sidebar on historical missions); the atmosphere stops most of the incoming x-rays. The first detection of x-rays from space (excluding those from the sun) was made on June 18, 1962. The rocket flight that lofted the instrument lasted for less than three minutes but found what is now known to be a neutron star with a low-mass companion star, called Sco-X1. Today x-ray astronomy has become the fastest-growing area of the field. From the largest structures to the smallest objects, x-ray observations have contributed key discoveries.
Warm Absorbers, Hot Controversy
It seems almost certain that most galaxies (our Milky Way included) have at their center a supermassive black hole several millions time the mass of our sun. Only in some of these galaxies, though, is this central black hole accreting enough matter to make the galaxy "bright," or active. In this case, the central region is called an active galactic nuclei, or AGN. Richard Mushotzky, an expert on AGN working at the NASA Goddard Space Flight Center, likes to point out that there is not even a basic theory of AGN, just a large numbers of observations that people are trying to understand. X-ray studies are the only available probes of the closest regions to the central black hole--and an area on which precise spectroscopy has recently shed some new light.
One class of AGN, called Seyfert I, has very bright optical emission lines and broad absorption lines in a spectra. Emission and absorption lines are an atom's signature, revealing information about the composition and the state of a gas. Different atoms emit radiation at different energies, so detecting an emission or absorption lines can, in principle, be used to deduce the composition of a gas. In practice, however, most atoms emit many lines, which can blend and be difficult to disentangle. For example, iron at a certain temperature emits radiation that is hard to separate from the contribution of neon.