20 APRIL 1998
"The Rossi Explorer was designed to probe closer than ever before the strongly curved space-time near neutron stars and black holes," said Frederick Lamb, a professor of physics and of astronomy at the University of Illinois at Urbana- Champaign. "These new results are based on the earlier dramatic discovery by Rossi that the brightness of many neutron stars varies more than a thousand times each second. These variations are the highest frequency oscillations ever detected in any astrophysical object."
Neutron stars are the dense cinders left behind when stars of about 10 times the mass of the sun explode in violent events called supernovae. Neutron stars have masses about the same as the sun but are only about 10 miles in diameter.
Consequently, the matter at their centers is much denser even than the matter in atomic nuclei. According to Einstein's theory of gravity, space-time near neutron stars is strongly curved.
Observation of the effects of strongly curved space-time would be the first confirmation of a strong-field prediction of general relativity.
Many neutron stars are found in binary systems with ordinary stars like the sun, but the stars orbit so closely that the neutron stars are devouring their companion stars. The strong gravitational field of the neutron star literally pulls gas off the surface of the companion star. The gas then spirals toward the neutron star.
The high-frequency brightness oscillations are thought to be caused by clumps of gas hurtling around the neutron star just above its surface at speeds approaching the speed of light, Lamb said.
When gas from these clumps collides with the surface of the star, the gas reaches temperatures of 100 million degrees and emits X-rays. The neutron star becomes brighter when the heated gas is on the side facing us and dimmer when the heated gas is on the other side.
Some of the neutron stars that produce high frequency X-ray oscillations radiate more energy in a second that the sun radiates in a week. These stars can be seen all the way across the galaxy, using X-ray telescopes like those on the Rossi Explorer.
"We had expected to see a cacophony of frequencies in the X-ray emission from this violent caldron of hot gas, somewhat like the discord that results when you press your hands randomly on a piano keyboard," Lamb said. "Instead, scientists using the Rossi satellite found that these neutron stars are playing cosmic chords, with two or three nearly pure tones."
"The clockwork of the universe is much more orderly than we had dreamed," Lamb said. "The pureness of these tones makes it possible to use them to investigate how matter moves in the strongly curved space-time near these neutron stars."
At the meeting, Lamb presented calculations carried out with Coleman Miller, a research scientist at the University of Chicago, and Dimitrios Psaltis, a research scientist at the Harvard-Smithsonian Center for Astrophysics.
These calculations showed how the X-ray brightness oscillations could be used to determine the masses and dimensions of neutron stars and to look for evidence of the innermost stable orbit, a key prediction of general relativity.
The innermost stable orbit is a qualitatively new prediction of Einstein's theory of gravity. According to Newton's theory, gas can orbit a compact star at any distance. In contrast, Einstein's theory predicts that if the star is sufficiently massive and compact, there is a region of space around it where space-time is so strongly curved that there are no stable circular orbits. Gas orbiting this close to the star unavoidably plunges to its surface.
The calculations of Miller, Lamb and Psaltis show that the frequency of the X- ray brightness variations should increase as the gas flow onto the neutron star -- and hence its X-ray power -- rises, until the clumps producing the oscillations are at the innermost stable orbit. At this point the oscillation frequency should become constant as the X-ray power continues to rise. A paper describing the team's results has been accepted for publication in the Astrophysical Journal.
William Zhang, a research scientist at NASA's Goddard Space Flight Center, presented new observations obtained with the Rossi Explorer that appear consistent with the effects predicted by Miller, Lamb and Psaltis. Zhang and his colleagues observed the neutron star called 4U 1820-30 over several months and found that as its X-ray power rises, the frequency of its brightness oscillation increases until it is oscillating about 1,050 times a second. As the X-ray power increases further, the frequency remains constant, indicating that the innermost stable orbit has been reached. The results obtained by Zhang's team also have been accepted for publication in the Astrophysical Journal.
"There is a good possibility that the Rossi Explorer has provided the first evidence supporting the predictions of Einstein's theory of gravity about how matter moves in strongly curved space-time," Lamb said. "All previous tests of general relativity have been made in regions where space-time is curved only very, very weakly. Searching for effects of strong gravitational fields is of fundamental importance. If this evidence for the existence of an innermost stable orbit is confirmed, it will be a major advance."
"Studying how matter moves in the strongly curved space-time near neutron stars also has allowed us to extract interesting new bounds on the masses and dimensions of these stars and on the stiffness of the superdense matter inside them," Lamb said. "The new evidence reported today suggests that the strong nuclear force is more repulsive than many nuclear physicists had expected and that the superdense matter in neutron stars is rather stiff."
While these new findings represent a very important and exciting development, they will require confirmation, Lamb cautioned.
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