Don Savage Headquarters, Washington, DC April 30, 1996 (Phone: 202/358-1727) Embargoed until 1 p.m. EDT Jim Sahli Goddard Space Flight Center, Greenbelt, MD (Phone: 301/286-0697) RELEASE: 96-81 NASA SPACECRAFT DISCOVERS FASTEST STELLAR VIBRATIONS YET Astronomers working with NASA's Rossi X-Ray Timing Explorer (XTE) spacecraft have discovered rapid fluctuations in the intensity of X-ray emissions from three unusual binary star systems that appear to be the signatures of the fastest vibrations ever detected in celestial objects. In one case, the oscillations reached frequencies as high as 1,130 times per second. The new findings are being reported today at a meeting of the High Energy Astrophysics Division of the American Astronomical Society in San Diego. The observations made using Rossi XTE offer scientists a new window on the strange physical conditions that scientists envision on neutron stars, which are believed to form when massive stars reach the ends of their lives and then explode as supernovas. The outer layers of a supernova are expelled into space, while its inner core remains and becomes a neutron star. Unlike normal stars, which are balls of hot gas, neutron stars are believed to possess solid crusts. The first detection of the remarkable fluctuations by Rossi XTE was made in February 1996. Astronomers led by Dr. Tod Strohmayer of the Universities Space Research Association (USRA) were observing the binary star 4U 1728-34, located in the general direction of the center of the Milky Way galaxy, in the constellation Sagittarius. This star pair was already well known to astronomers as a frequent source of powerful bursts of X-rays, which are thought to originate in hot gas that has streamed downward onto a very small and dense star known as a neutron star from a companion star. As the gas accumulates on the neutron star, it turns into a natural nuclear bomb, burning with a thermonuclear flash that produces a burst of X-rays lasting about ten seconds. Fortunately, 4U 1728-34 was in a bursting state when the Rossi XTE observations commenced. The astronomers were able to detect both the powerful bursts and the weaker "persistent" X-ray emission that is always emanating from the binary star. "We were very excited to catch several X-ray bursts in our first pointing at the object. We were even more excited when a quick look at the persistent X-rays data revealed very high frequency, nearly periodic oscillations which no one had ever seen before," said Strohmayer, who is stationed at NASA's Goddard Space Flight Center, Greenbelt, MD. "The observations seem to confirm long-standing theoretical ideas suggesting that physical conditions on a neutron star can change in less than one millisecond." (A millisecond is one-thousandth of a second.) The oscillations detected in 4U 1728-34 occurred at varying rates, reaching as high as 1,100 times per second. In subsequent Rossi XTE observations, investigators led by Michiel van der Klis of the University of Amsterdam, The Netherlands, have detected even faster oscillations in X-rays emitted by another binary star system, Scorpius X-1, which is named for the constellation in which it is located. Scorpius X-1 was the first object beyond the Solar System to be detected as a source of X-rays. In Scorpius X-1, the oscillations observed with the Rossi XTE have reached rates as high as 1,130 times per second. Further observations by the spacecraft's instruments have found oscillations of up to 900 times per second in a third binary star, 4U 1608-52, in the constellation Norma. Research on that star was led by Jan van Paradijs of the University of Alabama, Huntsville, and the University of Amsterdam, and by William Zhang of USRA, who is also from Goddard. Each of these three binary star systems contains a neutron star, and all of them are located in the southern sky. "It's possible that the oscillating X-ray emissions come from gas orbiting very close to the neutron star," according to Strohmayer. For example, material orbiting at ten miles above a neutron star would circle it about 700 times per second. "We have also measured a very periodic oscillation of 363 times per second during the bursts from 4U 1728-34. This may be the period at which the neutron star is spinning," he added. Other Rossi XTE data support this interpretation. "A more controversial possibility," he added, "is that we may be detecting for the first time the influence of waves on the surface of the neutron star or within its solid crust." Such waves occur in the gaseous layers of the Sun and other stars, but have not previously been found in neutron stars. "The detection of such waves might allow us to probe the unseen interiors of neutron stars, just as seismologists use earthquake waves to explore the inner layers of the Earth." The possibility that the Rossi XTE has detected actual waves in neutron stars or a very fast rotation period of one such star is of great scientific interest, said Jean Swank, Rossi XTE Project Scientist at Goddard and a collaborator in the research on all three binary star systems. If surface waves have been detected, that would be a scientific first. If a very fast rotation period has been detected in a neutron star in an X-ray binary system, the finding would tend to confirm a theory that certain very fast radio pulsars, known to be rotating neutron stars, are descended from fast-rotating members of X-ray binaries. In any case, "we have succeeded in one of our prime goals for this spacecraft, to detect and characterize rapid changes in celestial X-ray sources that may reveal their underlying physical conditions," Swank said. XTE was launched by a Delta II rocket on Dec. 30, 1995. Subsequently, NASA renamed it in honor of the late Professor Bruno Rossi of the Massachusetts Institute of Technology, Cambridge, one of the pioneers of X-ray astronomy. The Earth-orbiting spacecraft carries the largest X-ray detector yet flown in space, the Proportional Counter Array, which was developed at Goddard by Swank and her team members. -end- NASA press releases and other information are available automatically by sending an Internet electronic mail message to domo@hq.nasa.gov. In the body of the message (not the subject line) users should type the words "subscribe press- release" (no quotes). The system will reply with a confirmation via E-mail of each subscription. A second automatic message will include additional information on the service. NASA releases also are available via CompuServe using the command GO NASA.