X-ray vision beyond Superman's wildest dreams will soon be one step closer to reality with the completion of a powerful X-ray camera for viewing high-energy objects in our galaxy and beyond to the farthest reaches of the universe. "For the first time, we will be able to view the sky in X rays almost as clearly as we can view it from the largest optical telescopes and 10 times better than any X-ray images we have had before," says Gordon Garmire, the Evan Pugh Professor of Astronomy and Astrophysics at Penn State.
Garmire is the principal investigator who conceived and designed the camera, which is in its final stage of testing at the Marshall Space Flight Center in Huntsville, Alabama. "It has performed flawlessly, exceeding the most optimistic goals originally set for it in 1989 when NASA accepted the proposal for its flight," Garmire says.
The camera was built under Dr. Garmire's direction at the Center for Space Research at the Massachusetts Institute of Technology using sensors built under the direction of George Ricker at MIT's Center for Space Research using large-format CCDs developed at MIT Lincoln Laboratoriy in Lexington, Massachusetts. Its mechanical structure and power-conditioning-and-control unit were built at Lockheed Martin in Denver, Colorado.
The camera, the "AXAF Charge-coupled device Imaging Spectrometer" (ACIS), is one of two cameras slated for installation on the world's most powerful X-ray-astronomy observatory, NASA's Advanced X-ray Astrophysics Facility (AXAF), which is scheduled to be launched into space on the Space Shuttle in late 1998. The ACIS camera is a spectrometer that will record the energy of each X-ray that it detects from the high-energy objects as a unique amount of charge, convert the charge into a signal, and then send the spectral signals to scientists on Earth who will use the information to detect the presence of different elements.
AXAF will be the third of NASA's Great Observatories to be launched, following the Hubble Space Telescope, which detects ultraviolet, visible, and infrared rays, and the Compton Gamma-Ray Observatory, which detects gamma rays.
"X rays are the most useful energy band for studying high-energy phenomena associated with the most energetic objects in the universe," says Mark Bautz, a research scientist at MIT's Center for Space Research and a member of the ACIS development team. Scientists expect that the ACIS camera will reveal new information about the cycle of matter that ultimately made life on Earth possible. "We want to understand how stars produced the heavy elements like carbon and calcium, and how they ejected them into the interstellar medium where they eventually formed planets in star systems like our own," Garmire says.
The ACIS camera, when combined with the telescope's X-ray-focusing mirrors, has very high angular resolution, or sharpness of focus, which will allow it to see individual stars for the first time in regions where large numbers of stars are crowded. It also has high spectral resolution, which will give it the ability to determine the energy of individual X-rays over a wide range of X-ray energies.
"The ACIS camera is amazingly efficient in the way it responds to X rays," Ricker says. "It records images, photon-by-photon, in 50 X-ray colors simultaneously. In virtually no other part of the electromagnetic spectrum is it possible to do such a thing, and ACIS does so with near perfection."
"The channel of spectral information that we will get from AXAF will be a thousand times higher in energy than the light we can see," Garmire adds. Among the objects the camera is designed to see are massive black holes--100 million to a billion times the mass of the Sun--which are thought to be the power source at the heart of quasars, the most luminous known objects in the universe. The ACIS camera, because it is able to see very-high-energy radiation, may be able to detect the early growth of the seeds of quasars in the early universe. In addition, the ACIS camera will determine the temperature and distribution of hot gas in galaxies and clusters of galaxies--data scientists can use not only to measure their mass but also to estimate the mass of the entire universe.
Because high-energy X rays also can penetrate through dense clouds of dust like those that hide the center of the Milky Way, scientists hope the X-ray camera will be able to see clearly the heart of our own galaxy, which many astronomers suspect harbors a massive black hole.
"We also are interested in the mysterious gamma-ray bursts and their associated X-ray emissions from exploding surfaces of neutron stars, which are almost as massive as black holes, in the flares and coronae of stars that are a thousand times brighter than our Sun, and in other kinds of stars that have very-high-temperature gas around them," Garmire explains. Scientists also plan to study the earliest and latest stages of a star's life, which Garmire says are hard to see in the optical wavelengths, and Sun-like stars that could reveal what our own Sun might eventually do in the later stages of its life.
"The X-ray sky looks totally different from the optical sky, so things will look pretty unusual," Garmire says. For example, around supernovae remnants in our galaxy--stars that exploded several thousand years ago--Garmire expects to see "beautiful expanding hot bubbles of gas that we don't see with optical telescopes because this gas is too hot and tenuous to emit visible light."
The ACIS camera also will be able to see more "colors" with X rays--a much broader range of different wavelengths of energy than the human eye can see. "If we could see in X rays, we would see a more colorful universe with many unnamed colors that you can't imagine," Garmire says.
The camera will next be shipped to Ball Aerospace in Boulder, Colorado, for integration and testing, then to TRW in Redondo Beach, California, where the AXAF observatory is being assembled, integrated, and tested. In the summer of 1998, it is scheduled to be moved to the Kennedy Space Center in Florida, where it will be attached to an inertial upper stage (IUS) manufactured by the Boeing Company of Kent, Washington, and mounted into the Space Shuttle in preparation for its launch.
AXAF is being built for NASA's Marshall Space Flight Center in Huntsville, Alabama, by TRW in Redondo Beach, California. Science data collected by the observatory will be calibrated, processed, and distributed by the AXAF Science Center, which is operated for NASA by the Smithsonian Astrophysical Observatory in Cambridge, Massachusetts.