NASA SPACECRAFT DISCOVERS FASTEST STELLAR VIBRATIONS YET Astronomers working with NASA’s Rossi X-Ray Timing Explorer (XTE) spacecraft have discovered rapid fluctuations in the intensity of X-ray emissions from three unusual binary star systems that

NASA SPACECRAFT DISCOVERS FASTEST STELLAR VIBRATIONS YET Astronomers working with NASA’s Rossi X-Ray Timing Explorer (XTE) spacecraft have discovered rapid fluctuations in the intensity of X-ray emissions from three unusual binary star systems that appear to be the signatures of the fastest vibrations ever detected in celestial objects.

Don Savage, Jim Sahli

In one case, the oscillations reached frequencies as

high as 1,130 times per second. The new findings are being

reported today at a meeting of the High Energy Astrophysics

Division of the American Astronomical Society in San Diego.

The observations made using Rossi XTE offer scientists

a new window on the strange physical conditions that

scientists envision on neutron stars, which are believed to

form when massive stars reach the ends of their lives and then

explode as supernovas. The outer layers of a supernova are

expelled into space, while its inner core remains and becomes

a neutron star. Unlike normal stars, which are balls of hot

gas, neutron stars are believed to possess solid crusts.

The first detection of the remarkable fluctuations by

Rossi XTE was made in February 1996. Astronomers led by Dr.

Tod Strohmayer of the Universities Space Research Association

(USRA) were observing the binary star 4U 1728-34, located in

the general direction of the center of the Milky Way galaxy,

in the constellation Sagittarius.

This star pair was already well known to astronomers

as a frequent source of powerful bursts of X-rays, which are

thought to originate in hot gas that has streamed downward

onto a very small and dense star known as a neutron star from

a companion star. As the gas accumulates on the neutron star,

it turns into a natural nuclear bomb, burning with a

thermonuclear flash that produces a burst of X-rays lasting

about ten seconds.

Fortunately, 4U 1728-34 was in a bursting state when

the Rossi XTE observations commenced. The astronomers were

able to detect both the powerful bursts and the weaker

“persistent” X-ray emission that is always emanating from the

binary star. “We were very excited to catch several X-ray

bursts in our first pointing at the object. We were even more

excited when a quick look at the persistent X-rays data

revealed very high frequency, nearly periodic oscillations

which no one had ever seen before,” said Strohmayer, who is

stationed at NASA’s Goddard Space Flight Center, Greenbelt,

MD. “The observations seem to confirm long-standing

theoretical ideas suggesting that physical conditions on a

neutron star can change in less than one millisecond.” (A

millisecond is one-thousandth of a second.)

The oscillations detected in 4U 1728-34 occurred at

varying rates, reaching as high as 1,100 times per second. In

subsequent Rossi XTE observations, investigators led by

Michiel van der Klis of the University of Amsterdam, The

Netherlands, have detected even faster oscillations in X-rays

emitted by another binary star system, Scorpius X-1, which is

named for the constellation in which it is located.

Scorpius X-1 was the first object beyond the Solar

System to be detected as a source of X-rays. In Scorpius X-1,

the oscillations observed with the Rossi XTE have reached

rates as high as 1,130 times per second.

Further observations by the spacecraft’s instruments

have found oscillations of up to 900 times per second in a

third binary star, 4U 1608-52, in the constellation Norma.

Research on that star was led by Jan van Paradijs of the

University of Alabama, Huntsville, and the University of

Amsterdam, and by William Zhang of USRA, who is also from

Goddard. Each of these three binary star systems contains a

neutron star, and all of them are located in the southern sky.

“It’s possible that the oscillating X-ray emissions

come from gas orbiting very close to the neutron star,”

according to Strohmayer. For example, material orbiting at

ten miles above a neutron star would circle it about 700 times

per second. “We have also measured a very periodic

oscillation of 363 times per second during the bursts from 4U

1728-34. This may be the period at which the neutron star is

spinning,” he added. Other Rossi XTE data support this

interpretation.

“A more controversial possibility,” he added, “is that

we may be detecting for the first time the influence of waves

on the surface of the neutron star or within its solid crust.”

Such waves occur in the gaseous layers of the Sun and other

stars, but have not previously been found in neutron stars.

“The detection of such waves might allow us to probe the

unseen interiors of neutron stars, just as seismologists use

earthquake waves to explore the inner layers of the Earth.”

The possibility that the Rossi XTE has detected actual

waves in neutron stars or a very fast rotation period of one

such star is of great scientific interest, said Jean Swank,

Rossi XTE Project Scientist at Goddard and a collaborator in

the research on all three binary star systems. If surface

waves have been detected, that would be a scientific first.

If a very fast rotation period has been detected in a neutron

star in an X-ray binary system, the finding would tend to

confirm a theory that certain very fast radio pulsars, known

to be rotating neutron stars, are descended from fast-rotating

members of X-ray binaries.

In any case, “we have succeeded in one of our prime

goals for this spacecraft, to detect and characterize rapid

changes in celestial X-ray sources that may reveal their

underlying physical conditions,” Swank said.

XTE was launched by a Delta II rocket on Dec. 30,

1995. Subsequently, NASA renamed it in honor of the late

Professor Bruno Rossi of the Massachusetts Institute of

Technology, Cambridge, one of the pioneers of X-ray astronomy.

The Earth-orbiting spacecraft carries the largest X-ray

detector yet flown in space, the Proportional Counter Array,

which was developed at Goddard by Swank and her team members.

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