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
“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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