Don Savage Headquarters, Washington, DC October 6, 1995 (Phone: 202/358-1547) Jim Sahli Goddard Space Flight Center, Greenbelt, MD (Phone: 301/286-0697) George Diller Kennedy Space Center, FL (Phone: 407/867-2468) RELEASE: 95-162 NASA's X-RAY TIMING EXPLORER TO STUDY THE VIOLENT UNIVERSE X-rays, recognized mostly for uses in medical equipment and airport security machines, soon will help scientists peer into some of the most exotic and violent objects in the universe. Among the objects to be studied by NASA's X-Ray Timing Explorer (XTE) mission, set for launch Nov. 6, 1995, are stellar black holes, neutron stars and quasars. Stellar black holes and neutron stars represent the end stages of the lives of stars several times heavier than the Sun. Having exhausted their nuclear fuel, these stars collapse into an incredibly dense object roughly the size of Manhattan Island. Quasars are thought to be super-massive black holes about the size of the solar system, containing the mass of a billion stars. The enormous gravity of these objects suck in gas and dust from nearby stars and space. Before disappearing into the object, this infalling material is heated to eighteen million degrees Fahrenheit or more. This chaotic and violent process produces an X-ray output that varies tremendously on virtually all time scales from less than a thousandth of a second to several years. The most advanced observatory of its type ever flown in space, XTE may actually "write the textbooks" on these energetic and violent sources of X-rays. XTE's unique capabilities will allow astronomers to observe in detail their X-ray variations on their shortest natural time scales -- as short as one one-millionth of a second. -more- -2- XTE will carry a scientific payload of three instruments, the Proportional Counter Array (PCA), the High Energy X-ray Timing Experiment (HEXTE) and the All Sky Monitor (ASM). The PCA and HEXTE are the heart of the observatory, aligned to observe the same source simultaneously. The wide-field ASM will serve a very important role in scanning the entire sky on a virtually continuous basis to alert astronomers to major outbursts from previously undetected sources, known as "X-ray novae," as well as significant changes in known sources. The spacecraft can be rapidly maneuvered in response to such events to allow more detailed observations by the PCA and HEXTE instruments. The ASM was developed by the Massachusetts Institute of Technology, the PCA by NASA's Goddard Space Flight Center, and the HEXTE by the University of California, San Diego. Goddard integrated and tested the spacecraft and manages the XTE project for the Office of Space Science, NASA Headquarters, Washington, DC. The 6,700-pound XTE spacecraft will be launched on a McDonnell Douglas Delta II rocket from Cape Canaveral Air Station, FL, into low-Earth orbit at an altitude of 362 miles and an inclination of 23 degrees. A two-year prime mission is scheduled. X-Ray Astronomy XTE will not use X-rays to look into the interiors of astronomical objects as do machines in hospitals and airports. Instead, XTE's instruments will give a more complete understanding of the extremely violent processes which produce X-rays in the cosmos, such as rapidly spinning neutron stars or black holes sucking material from a companion star, massive black holes lurking at the center of incredibly powerful and compact quasars, and other exotic phenomena. One of the chief characteristics of X-rays produced by such compact objects is that they vary in intensity over time. The intensity of X-rays emitted by spinning neutron stars, for example, appears to vary at regular intervals due to the stars' rotation. This effect is similar to the bright pulse of light seen from a lighthouse -- as the neutron star rotates, its beam of energy flashes past us with each revolution. These sweeping pulses of energy are a direct indicator of how fast a neutron star is spinning. Some neutron stars are observed to rotate faster than 30 times a second despite a mass at least 40 percent greater than that of the Sun. By studying the very short time scale variations in the X-rays emitted in the death spiral of this material, XTE will attempt to confirm the existence of black holes in the Milky Way galaxy and elsewhere in the universe, and provide valuable new information on the physical processes taking place in their vicinity. - 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. Questions should be directed to (202) 358-4043.