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Mysterious gamma-ray star reveals clues to its nature
Posted: June 9, 2000

An artist's concept depicts the magnetic field lines rising from the surface of a magnetar. Photo: NASA
Astronomers have spotted the dance of what may be energetic protons or iron atoms in a strange and rare class of objects known as soft gamma-ray repeaters (SGR). This observation, the first emission line spectrum for an SGR, may lend credence to the theory that these objects are highly magnetized neutron stars, capable of swiping clean a credit card and sucking pens out of pockets at a distance 100,000 miles away, or half the distance to the moon.

Dr. Tod Strohmayer of NASA's Goddard Space Flight Center and Alaa Ibrahim, a George Washington University graduate student, used NASA's Rossi X-ray Timing Explorer (RXTE) to identify a specific emission line at the 6.4 keV energy band in SGR 1900+14. They presented their results Thursday at the American Astronomical Society meeting in Rochester, N.Y., and in an upcoming article in the Astrophysical Journal Letters.

"Astronomers have debated about the nature of SGRs ever since their discovery in 1979," said Strohmayer. "For almost twenty years, the exact nature of these objects remained a big mystery. But in the last few years we have learned many new things about them, and many of these new findings have supported the idea that SGRs are magnetars, a theorized, highly-magnetic kind of neutron star. Our observations support the magnetar model, although they don't yet completely rule out other possibilities."

SGRs emit a series of enormous bursts of X rays, often hundreds of flashes that last only a second over a period of a couple of weeks to months. Then the bursting disappears for months or years before the SGR becomes active again. An SGR burst can release as much energy in a single second as the Sun does in a whole year. There are only four known SGRs, and they all lie within our galaxy or in the Large Magellanic Cloud, a neighboring galaxy.

SGRs are relatively young (less than 10,000 years old) and spin slowly, unlike the Crab and other young pulsars, which spin hundreds of times faster. Also, SGRs randomly emit bursts of soft gamma rays (synonymous with "hard" or high-energy X rays), uncharacteristic of X-ray pulsars. Pulsars, in this context, are spinning neutron stars -- the collapsed remains of a star once several times larger than the Sun -- which emit pulses of light at extremely consistent time intervals.

Believed magnetars. Photo: NASA
In 1992, astrophysicists Robert Duncan and Christopher Thompson suggested that SGRs were super-magnetized neutron stars -- a million billion times more magnetic than the Sun -- which they dubbed magnetars because they are a thousand times more magnetic than the already potent neutron star. The magnetar's strong magnetic field would act like stellar brakes on the ordinarily rapid spin process, explaining why such young objects spin so slowly. Another theory posits that SGRs are weakly accreting pulsars, either alone or in a binary star system. Very little matter falls onto the pulsar, so we do not see much activity.

With RXTE, Strohmayer and Ibrahim observed a 6.4 keV emission line in SGR 1900+14. ("keV" stands for kiloelectron volts, a unit of energy.) An emission line spectrum is a jagged plot that looks somewhat like an electrocardiogram. Specific elements, such as iron, can emit and absorb specific X-ray energies and reveal their presence by sharp peaks and valleys in such a plot. An emission line at the 6.4 keV energy level can denote the presence of iron or of a proton circulating in the magnetars' powerful magnetic field.

What's interesting about SGR 1900+14 is that it experienced a giant flare two days before the burst that Strohmayer and Ibrahim observed, likely caused by a realignment of the magnetic field. This was only the second flare of its kind ever detected from an SGR.

In one scenario, this flare before the burst could have flung plasma (atoms with missing electrons) off the surface of the SGR and into the magnetosphere, the region around the SGR bounded by the magnetic field. In the highly energized magnetosphere, larger atoms would be ripped apart into smaller ones, such as hydrogen, which is just a proton. The burst that came a few days later acted like a lamp that revealed these protons falling back to the surface and releasing X rays at 6.4 keV. This range of energy, for protons, matches what would be expected in the environment of a magnetar.

TSGR 1900+14 as seen by Japan's ASCA spacecraft.
In a second scenario, the flare could have flung plasma past the magnetosphere. Large atoms such as iron, far from the terror of the magnetosphere, would not break down into hydrogen. The burst that followed illuminated the iron atoms, which emitted X rays at 6.4 keV. This energy range is typical for non-magnetized iron. Yet only a magnetar environment could produce conditions that would throw iron atoms into space in this fashion.

A third scenario, however, allows for the "accretion" theory of SGR origin, as opposed to the magnetar theory. The 6.4 keV emission line is typical of iron in accreting systems, where a black hole or neutron star steals globs of gas from a nearby, healthy star. The gravity from the black hole or neutron star supplies the power, in the form of gravity; and the healthy star supplies the iron. No strong magnetic field is needed.

Strohmayer and Ibrahim said that other factors favor the first and second scenarios over the third. They discuss these factors in their paper.

"The emission line sheds new light on the nature of SGRs," said Ibrahim. "It provides us with a better understanding of their environment and could be used to estimate their magnetic field, thus allowing us to test the magnetar model and other proposed theories for these mysterious objects."

RXTE was built at Goddard with university collaboration and launched by NASA in December 1995 to observe fast-changing, energetic and rapidly spinning objects, such as supermassive black holes, active galactic nuclei, neutron stars and millisecond pulsars. The AAS meeting, held from June 4-8, features over 800 scientists and 500 scientific papers.