Satellite may help find answers to primary questions of astrophysics

By Jim Barlow

This month's launch of a satellite carrying sophisticated X-ray technology
offers scientists a chance to answer "some of the most interesting and
fundamental questions of modern astrophysics," says a UI researcher who is
a scientific spokesman for the mission.

NASA's $197 million "X-Ray Timing Explorer" (XTE) satellite - tentatively
scheduled for a Nov. 20 launch from the Kennedy Space Center - will study
the X-rays produced by white-dwarf stars, neutron stars and black holes,
the densest objects known in the universe. Scientists hope that data from
the satellite will firmly establish the existence of black holes, said
Frederick Lamb, a UI professor of physics and astronomy. He has been
involved in the mission's planning since 1979.

"These objects are the final end states of matter in the universe," Lamb
said. "They are of special interest because they represent the end of the
journey that began with the Big Bang. It is believed that these objects
co-exist with us today; that they are not ancient history."

White dwarfs form when stars with masses about the same as the sun's
exhaust their thermonuclear fuel, contract to about the size of Earth and
cool. Neutron stars have solid outer crusts and a liquid core; they are
thought to form when stars with 10 times the mass of the sun collapse and
explode, leaving behind a cinder that contains the densest matter known in
the universe. Black holes are believed to form when stars with masses much
greater than that of the sun exhaust their fuel and collapse inward,
"trapping matter and radiation forever," Lamb said.

When nearby matter falls onto white dwarfs, neutron stars or black holes,
it is heated to temperatures of billions of degrees and radiates mostly
X-rays. Because of Earth's atmosphere, the
X-rays cannot reach the ground, so the only way to study the X-rays is in
space. The XTE satellite will carry complementary instruments - built by
the University of California at San Diego, the Goddard Space Flight Center
in Maryland and the Massachusetts Institute of Technology - that will seek
out and study X-rays from these objects, which produce very little visible
light.

Lamb's UI team will analyze the X-rays from neutron stars and possible
black holes.

"Some neutron stars have extremely strong magnetic fields and are thought
to rotate thousands of times a second, producing rapidly rotating X-ray
beams," Lamb said. "We hope XTE will allow us to detect the spins of these
stars for the first time and to study what happens as matter falls into
them. We may also be able to time the orbital motion of matter falling into
possible black holes. If so, we can compare that timing with the
predictions of Einstein's theory of gravity, and if they match up, that
will be very strong evidence that the object is indeed a black hole."

A near-real-time map of the X-ray sky produced by the satellite will be
accessible to the public as well as scientists on the Internet during the
mission at this World Wide Web address:
http://heasarc.gsfc.nasa.gov/docs/xte/xte_1st.html