A new X-ray in the sky.

By the end of this month, Japanese scientists expect to launch an X-ray observatory that promises to shed new light on some of the more intriguing denizens of the universe: supernovas, neutron stars, and black holes. Using a suite of four telescopes, each with its own spectrometer, the Japanese-U.S. satellite will image medium-energy X-ray emissions from 300 to 12,000 electron-volts. In addition, the spectrometers will analyze the energy and intensity of the X-ray light by separating the radiation into its component wavelengths.

Two of the spectrometers feature electronic photon detectors known as charged-coupled devices, the first time that astronomers have used these sensitive detectors to analyze medium-energy X-ray emissions, notes the observatory's deputy project scientist, Nicholas White of NASA's Goddard Space Flight Center in Greenbelt, Md.

Known as Astro-D until launch, when it will be renamed, the observatory is expected to operate for five years. In recording radiation from atoms of iron, silicon, sulfur, and oxygen, the observatory will chart the course of stellar violence. These atoms are produced in the cosmic cauldron created when massive stars die an explosive death as supernovas. Thus, detecting X-ray emissions from iron may help track the evolution of supernovas and detail what happens when a supernovas expanding blast wave slams into its surroundings.

Astro-D will also gather data to "weigh" the mass of suspected black holes, says White. As material spirals into a black hole, it emits a last gasp of X-rays. These X-rays appear redshifted -- shifted to longer wavelengths - because of the gravitational tug exerted on the photons by the massive black hole. Indeed, the more massive the hole, the greater this gravitational redshift. So if the observatory's spectrometers can accurately measure the gravitational redshift of X-rays emitted near a black hole, astronomers will be able to calculate the mass of the compact object that lurks there.

Astronomers also hope to use the satellite to more accurately infer the amount of dark matter - invisible material that doesn't glow like ordinary matter, yet exerts a gravitational force -- among clusters and small groups of galaxies. By mapping the temperature and amount of X-ray-emitting gas in a galaxy cluster, the satellite enables researchers to calculate the total amount of mass that the cluster must possess in order to hold the gas in place. If the visible mass of the cluster isn't enough to do the job, then the cluster must contain a large percentage of hidden, or dark, matter, researchers reason.

Such measurements have already been performed with a lower-energy X-ray satellite called ROSAT. Researchers headed by Richard F. Mushotzky of Goddard reported last month that, based on the size and energy of its X-ray-emitting gas, the small galaxy group NGC 2300 contains 10 to 20 times as much dark matter as visible matter, one of the highest ratios ever inferred (SN: 1/9/93, p.20).

The researchers hope to use the newly launched satellite to improve their measurements of this galaxy group. In addition, Astro-D can study X-ray emissions in fainter, more distant collections of galaxies, White says. If the observatory finds an equally high ratio of dark matter in many other small groups, then such collections may represent repositories for much of the dark matter in the universe, Mushotzky notes.