MEDIA RELATIONS OFFICE
JET PROPULSION LABORATORY
CALIFORNIA INSTITUTE OF TECHNOLOGY
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http://www.jpl.nasa.gov
Contact: Diane Ainsworth
FOR IMMEDIATE RELEASE
October 1, 1998
ULYSSES CAPTURES GAMMA-RAY FLARE FROM SHATTERED STAR
The signal of a cataclysmic magnetic flare emanating from a
star that cracked apart halfway across the galaxy has been
captured by NASA's Ulysses spacecraft and is providing important
clues about some of the most unusual stars in the universe.
The magnetic burst from the star SGR1900+14, located in the
constellation Aquila 20,000 light-years away, was observed by
Ulysses and other spacecraft with high-energy radiation detectors
in space on August 27, 1998, as its heavy metal crust fractured
and released the most powerful wave of gamma radiation yet
observed from this type of star.
"Soft gamma repeaters (SRGs) emit magnetic radiation
sporadically, every few years, unlike gamma ray bursts, which
explode and disappear," said Dr. Edward J. Smith, Ulysses project
scientist at NASA's Jet Propulsion Laboratory, Pasadena, CA.
"This was the fourth soft gamma repeater to be observed, but
unlike the others we have studied, this one emitted an
exceedingly intense burst of radiation. We estimate that it
released as much energy in a few seconds as the Sun emits in 300
years." Ulysses is a joint mission of NASA and the European
Space Agency.
SGR1900+14 is a newly discovered type of star called a
"magnetar" - a dense ball of super-heavy matter about the size of
a city, but weighing more than the Sun. Objects in this class
have the greatest magnetic fields known in the universe. A
magnetar is so intense that it powers a steady glow of X-rays
from the star's surface, often punctuated by brief, intense
gamma-ray flashes and, occasionally, by catastrophic flares like
the one observed on August 27. Astronomers think that all these
effects are caused by an out-of-control magnetic field -- one
capable of heating, mixing and sometimes cracking the star's
rigid surface.
Using several spacecraft detectors, including the Ulysses
gamma ray burst instrument, scientists were able to measure this
extremely rare event and pinpoint the precise source of the
explosion with unprecedented clarity.
"The star, which has an extremely strong magnetic field,
appears to have experienced a 'star quake' so powerful that it
created a temporary ionosphere on the night side of Earth and
sent two spacecraft into protective safe modes," Smith said.
Data from the Ulysses experiment, showed radiation counts
that rocketed from background (near zero) levels to several
thousand electrons per second. Dr. Kevin Hurley of the
University of California, Berkeley, who is principal investigator
of the gamma ray burst experiment on Ulysses, reported that
energy measurements were two times greater than any other
recorded burst.
"The radiation, as seen by the gamma ray burst detector,
spiked quickly and soon settled into a series of ever-smaller
spikes that clearly revealed the neutron star's rotational
period," Hurley reported at a NASA science press briefing on
September 29. "The star reminded us of a dying lighthouse. It
kept rotating, but the lamp steadily faded away."
Hurley, who had been part of a team observing the star,
recorded pulses or flashes of magnetic radiation emanating from
the star every 5.16 seconds using another satellite, known as the
Japanese/NASA Advanced Satellite for Cosmology and Astrophysics
(ASCA). Comparisons of the ASCA data and measurements from other
satellites showed that the X-ray pulses were gradually slowing
down after the radiation burst subsided.
From its intensity and rotational slowing, scientists
calculated that SGR1900+14 has a magnetic field about a thousand
trillion times stronger than Earth's magnetic field and about one
thousand times stronger than any found elsewhere in the universe,
Smith said. During the flashing episode, Dr. Chryssa Kouveliotou
of NASA's Marshall Space Flight Center in Huntsville, AL, who led
another team observing the star with sensitive X-ray detectors
aboard NASA's Rossi X-ray Timing Explorer satellite, found faint
X-rays coming from the star, similar to what they had observed in
another soft gamma repeater which turned out to be a magnetar.
Three of the four confirmed soft gamma repeaters -
designated 1900+14, 1806-20 and 0526-66 -- have localized X-ray
emissions; 1806-20 and 1900+14 have regular pulsations and 0526-
66 had an eight-second period during its magnetic explosion
observed in 1979. It is by comparing the change in the
rotational period of these stars across several observations that
scientists can measure their magnetic fields.
"Magnetars seem to answer several mysteries about the
structure and evolution of stars," said Kouveliotou. "We think
magnetars spend their first 10,000 years as soft gamma repeaters.
As they weaken with age and slow their rotation, they become
anomalous X-ray pulsars -- stars that do not have enough 'juice'
to flash anymore, but which emit a steady flow of X-rays for
perhaps another 30,000 years. After that, they fade to black and
drift for eternity through the heavens. The absence of
observable pulsars in some supernova remnants just means that the
pulsar's lights have gone out sooner than we expected."
Additional information on magnetars or the August 27 burst
is available on the Internet at
http://www1.msfc.nasa.gov/NEWSROOM/ and
http://www.magnetars.com/
The Ulysses mission to study the poles of the Sun is managed
jointly by NASA and the European Space Agency. The Jet
Propulsion Laboratory manages the U.S. portion of the mission for
NASA's Office of Space Science, Washington, DC. JPL is a
division of the California Institute of Technology, Pasadena, CA.
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