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Astrophysicists announced that the discovery of very rapid oscillations in the brightness of some X-ray-emitting neutron stars has yielded important new constraints on the properties of the superdense matter at the centers of these stars. The data also may represent the first evidence for a unique effect of strongly curved space-time predicted by Einstein's theory of gravity but never before observed.^76d These new results are based on the earlier dramatic discovery by the Rossi X-Ray Explorer that the brightness of many neutron stars varies more than a thousand times each second. 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. These variations are the highest frequency oscillations ever detected in any astrophysical object. 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. When gas from these clumps collides with the surface of the star, the gas reaches temperatures of 100 million degrees and emits X-rays. 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.

"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." 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 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.

* 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."

CHAMPAIGN, Ill. -- A team of astronomers studying supernova remnants has found direct evidence linking the pattern of their X-ray emission to the size -- and therefore, age -- of the remnants.

"As supernova remnants grow in size, we discovered that the X-ray emission from the center of the remnant becomes brighter than that from the edge," said Rosa Williams, a graduate student in astronomy at the University of Illinois. "Our results provide new insight into the distribution of gas shocked by the supernova blast wave, and the nature of material between stars."

When a massive star nears the end of its life, the outward pressure of its thermonuclear reactions can no longer counter the inward pull of gravity. The star collapses in on itself and, within a fraction of a second, rebounds in a tremendous explosion called a supernova. Much of the star's material is flung outward, forming an expanding sphere of gas and dust called a supernova remnant. X-rays are produced by supernova remnants when the supersonically expanding shock hits the surrounding material.^76h

"Since the shock front is roughly spherical, we normally expect the X-rays to form a spherical shell structure, like a soap bubble," Williams said. "The shell appears brightest at the edge, where our line of sight intersects the most X-ray-emitting material. While the shocked material in the interior of the shell can also produce X-rays, this gas is usually too diffuse to be detected."

Nonetheless, it has long been known that some supernova remnants show X-ray emission over the entire face of the remnant, sometimes even brighter toward the center, Williams said. "If this emission is indeed coming from inside the remnant, where the shock has already passed, then something must be raising the density of the material in the interior until the hot gas is dense enough, and therefore bright enough at X-ray wavelengths, to be seen by our instruments."

Using X-ray observations from the ROSAT satellite, Williams and her colleagues -- You-Hua Chu and John R. Dickel of the U. of I., Robert Petre of the Goddard Space Flight Center, R. Chris Smith of the Cerro-Tololo Inter-American Observatory, and Maritza Tavarez of the University of Michigan -- studied the structure and distribution of gas in supernova remnants within the Large Magellanic Cloud, a satellite galaxy of our own Milky Way. They classified the objects by their X-ray structures, and determined that these structures vary according to the remnant's size: Smaller remnants have shell-like X-ray emission, while larger remnants appear to have more X-ray emission from inside the shell.

"Because supernova remnants are continually expanding, a remnant becomes larger as it gets older," Williams said. "So it looks like what we're seeing is a change in the remnants as they age, with more and more of the X-ray emission coming from the inside of the remnants."

If this is indeed the case, said Williams, who presented the team's findings at the January meeting of the American Astronomical Society, theories to explain the presence of detectable X-ray emission from the interior of supernova remnants must also account for the age progression.

Pulsars that only emit X-rays, once considered "anomalous", now officially outnumber those that emit radio waves. This is leading astronomers to rethink their ideas about what happens after a typical dying star explodes as a supernova.

A supernova explosion occurs when a star runs out of nuclear fuel and shrinks catastrophically under its own gravity. The result is a super-dense neutron star about the size of Mount Everest. Current theories of how these stars behave predict that they should all act as radio pulsars, sweeping a narrow beam of radio waves around the sky like a lighthouse a hundred times a second. So why didn't radio searches find more pulsars in supernova remnants?

"The trouble is that hardly more than 1 per cent of the 300-odd known young supernova remnants contain associated radio pulsars," says Eric Gotthelf of NASA's Goddard Space Flight Center near Washington DC. But now the reason they went missing is clear, Gotthelf says. Astronomers were looking in the wrong part of the electromagnetic spectrum.

In the past few years, astronomers using the Japanese-American ASCA satellite have found three "point-like" objects in the centres of supernova remnants which are emitting pulses of X-rays. Now Gotthelf says that he has just picked out three more of these "anomalous X-ray pulsars" (AXPs) in X-ray sources observed by the satellite. Add these three to the list and anomalous pulsars in supernovae remnants will outnumber the four known radio pulsars associated with supernovae remnants for the first time, he reports in a paper to appear in the journal Memorie della Societá Astronomica Italiana.

These findings will mean that radio pulsars are the exception rather than the norm. "This is a complete reversal of our thinking," says Gotthelf. David Hough of Trinity University in San Antonio, Texas, confirms that these new findings mean that "the book on how pulsars are born in supernovae may have to be rewritten".

The X-rays coming from AXPs are produced by matter channelled by the star's magnetic field lines and heated to enormous temperatures. AXPs spin a thousand times slower than radio pulsars and are slowing down rapidly. This is puzzling, because when a star shrinks to the relatively tiny size of a neutron star it should automatically spin very fast. According to Gotthelf, the most likely explanation is that AXPs are indeed born spinning fast, but slow down quickly because they have a super-strong magnetic field, hundreds of times stronger than in radio pulsars.

"Such a strong magnetic field would drag material around as the star spins, sapping the star of rotational energy," says Gotthelf. A super-strong magnetic field would also prevent the formation of the electrons needed to produce radio waves. One possible explanation for the different magnetic field strengths in radio pulsars and X-ray pulsars is that stars start out with a natural variability in magnetic fields before they collapse.

At least two more sensitive X-ray satellites will be launched in the next few years, and Gotthelf believes they will find many more radio-quiet pulsars.