Goddard Space Flight Center, Greenbelt, Md.
August 24, 2000
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."
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.
Official version of the published article.
Astronomische Persberichtendienst
21 augustus 2000
Neutronensterren zijn de resten van zeer zware sterren. Aan het einde van hun leven exploderen ze, waarbij de buitenste lagen de ruimte worden ingeblazen en de kern instort tot een neutronenster. Een neutronenster heeft een diameter van ongeveer twintig kilometer, draait enkele tientallen tot honderden keren per seconde om zijn as en heeft een massa gelijk aan onze zon. Een theelepeltje neutronenster weegt ongeveer honderdmiljoen ton.
Door de enorme dichtheid van de materie heeft een neutronenster een intens zwaartekrachtsveld. Dit veld is zo sterk dat een neutronenster in staat is om de buitenste lagen van een naburige ster op te slokken. Het gas van deze ster wordt in een platte schijf rondom de neutronenster getrokken. Hoe dichter het gas bij de neutronenster komt, hoe hoger de temperatuur wordt. Dit kan oplopen tot ver boven de miljoen graden Celcius. Als het gas met bijna de lichtsnelheid op het oppervlakte knalt, ontstaat er röntgenstraling. Dit is het hoofdsignaal wat gemeten wordt.
Het rondzwiepende zwaartekrachtsveld van de neutronenster sleurt de ruimte als het ware met zich mee en is daardoor boven en onder de evenaar anders vervormd dan bij de evenaar . De vervorming van de ruimte maakt dat de schijf gaat slingeren en dat de dikte van de schijf variëert Daardoor valt niet alle materie precies op de evenaar maar ook daar boven en daar onder. Het verschil in de kromming van de ruimte zien de Amsterdamse astronomen terug in de oscillaties van de röntgensignalen. Met de ontdekking van de vervorming in deze oscillaties heeft men een nieuw aspect waargenomen van de manier waarop materie zich in een sterk gekromde ruimte gedraagt. De verwachting is dat dit verschijnsel ook bij het enorme zwaartekrachtsveld van zwarte gaten aangetoond kan worden.
Bovenstaand onderzoek maakt deel uit van het programma van de Nederlandse Onderzoekschool Voor Astronomie (NOVA). NOVA is een samenwerkingsverband tussen de astronomische instituten van de universiteiten van Amsterdam, Groningen, Leiden en Utrecht.
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