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Posted: August 27, 2000

An image shows one of the Lense-Thirring precessional motions discovered by Markovic and Lamb in 1998. The accretion disk (colored orange) is composed of hot, swirling gas that is slowly spiraling toward the black hole (center).
This just in: Scientists have uncovered sets of oscillating X-ray signals from three neutron stars that may tell the story of the bending of the very fabric of space around these objects, broadcast to us from the stars much like the details of a science talk show buried within oscillating AM radio waves.

With AM radio, rapid changes in the strength of the radio wave signal (a modulation of amplitude) carry the encoded radio program.  The modulation appears as "sidebands," weaker signals on either side of the oscillating carrier wave. In three neutron stars, the first discovered sidebands in X-ray emission transmit a different kind of story: details about the stars' mass and spin, the distortion of space-time predicted by Albert Einstein, and the location of the inner-most stable orbit around each star -- all encoded by a natural process.

Three scientists at the Astronomical Institute of the University of Amsterdam -- Drs. Peter Jonker, Mariano Mendez (now with the Observatorio Astronomico La Plata in Argentina) and Michiel van der Klis -- used NASA's Rossi X-ray Timing Explorer to uncover these modulations caused by gravity at its extreme. Their work appears in an article published today in The Astrophysical Journal Letters.

"For a couple of years now with the Rossi Explorer we have seen very rapid oscillations in the brightness of X-ray-emitting neutron stars, evidence of a strong curvature in space-time," said Jonker. "Now we are seeing sidebands, another set of oscillations that provide even more detail about this world of extreme gravity. This is important new information needed to describe the environs of these fascinating objects."

According to Einstein's theory of gravity, space-time near a neutron star (as well as a black hole) is strongly curved. The  discovery of sidebands in the X-ray emission allows new tests to see whether Einstein was right.

Neutron stars are the dense cinders left behind when certain massive stars explode in violent events called supernovae. A neutron star contains about the same mass as the Sun squeezed into a sphere about 10 miles in diameter. Such a dense object exerts a tremendous gravitational force that, when part of a binary star system, is capable of pulling in gas from the neighboring star. This gas spirals onto the neutron star via an orbiting swirl called an accretion disk, which is visible in many wavelengths, particularly in X-rays.

"Einstein's theory of how matter moves in strongly curved space-time has not yet been verified," said van der Klis. "Previous measurements have been made only where gravitational fields are weak, such as in our solar system. The Rossi Explorer is the first instrument that has allowed us to actually see how matter moves in the strong gravitational field near a neutron star. X-rays carry that message."

An artist's concept of the RXTE satellite. Photo: NASA
The signals that the Rossi Explorer is capturing are high-frequency oscillations in the X-ray emission, likely produced by clumps of gas in the accretion disk that are hurtling around the neutron star just above its surface at nearly the speed of light.

"The discovery of rapid X-ray oscillations using the Rossi Explorer a few years ago launched a swirl of intense theoretical work that has produced several possible explanations," said Dr. Jean Swank, an X-ray astronomer at NASA's Goddard Space Flight Center and the project scientist for the Rossi Explorer mission. "The newly discovered signals may be the key that unlocks the door, so we can see what the right explanation is."

Several theorists have already suggested that the newly discovered sideband emission from gas orbiting around the three neutron stars -- named 4U 1608-52, 4U 1728-34, and 4U 1636-53 -- may be explained by Lense-Thirring precession of the gas. This refers to the dragging of inertial frames, a qualitatively new prediction of Einstein's theory of gravity. Drs. Fred Lamb and Draza Markovic of University of Illinois at Urbana-Champaign (UIUC) have published work showing that Lense-Thirring precession of an accretion disk can persist and might be visible; animation based on their calculations is referenced below.

Lamb compares orbiting gas and the dragging of inertial frames around a neutron star to a marble rolling down a large funnel covered with fabric. "A dense neutron star is like a heavy, spinning object at the center of this funnel," Lamb said. "As the object turns, it will twist the fabric, stretching it. The marble will start to roll toward the center of the funnel, but it soon deviates to the side because the fabric is moving and carries the marble with it."

In a similar fashion, clumps of gas falling inward will be dragged around a spinning neutron star. If, as expected, the inner part of the disk is slightly tilted with respect to the star's spin-axis, the X-ray emission produced when the gas collides with the star will vary with a frequency equal to twice the frequency at which inertial frames are dragged around the star. This variation will produce sidebands on the primary oscillation, similar to those observed.

If further analysis shows that this is indeed the case, the newly discovered sidebands will be direct evidence of frame dragging and will establish that the frequencies of the primary X-ray oscillations are indeed the orbital frequencies of gas hurtling around these neutron stars. Further study of the sidebands will provide valuable new information about the effects of strong gravity and the properties of extremely dense matter, two of the most fundamental outstanding questions in modern astrophysics.

"This latest discovery demonstrates again the unique capabilities of the Rossi Explorer to probe the properties of the strong gravitational fields near neutron stars and black holes," said Lamb. "The Rossi Explorer carries the largest X-ray detectors ever flown in space and was designed specifically to measure the motion of hot gas near compact objects. With each new discovery by the Rossi Explorer, we are coming closer to pinning down the properties of space-time near these bizarre objects."

The Rossi X-ray Timing Explorer is operated from Goddard Space Flight Center in Greenbelt, Md. Launched in 1995, the spacecraft was developed by Goddard with Massachusetts Institute of Technology and University of California, San Diego. Rossi observations are proposed by the international X-ray Astronomy community.