Even the "soft" stars have a "hard" side

Soft Gamma Repeaters can erupt
like distant Gamma-Ray Bursts

Dec. 10, 1999: Soft-gamma repeaters have a hard side. It's hard enough that they could almost be mistaken for the hard gamma-ray bursts that come from deep in the observable universe.

Right: An artist's concept depicts the magnetic field lines rising from the surface of a magnetar, and the plasma clouds around the star. Credit: Dr. Robert Mallozzi, University of Alabama in Huntsville.

Early in the 1980s, astrophysicists realized that they were looking at two distinct classes of gamma-ray burst events. One class came from within or near our galaxy and would put in repeat performances at unpredictable intervals. The other class came from outside our galaxy and did not repeat.

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In 1987, the science community put a name on the first class: Soft Gamma Repeaters, or SGRs. The name was selected partly because it sounded better than the alternative, Hard X-ray Repeaters (there is no hard delineation where one part of the spectrum stops and the other starts -- usually a spectral band is marked by the capabilities of an instrument). The other class was simply called gamma-ray bursts or GRBs. Only in the last few years have scientists been able to establish that they come from galaxies several billion light years away, and thus occurred during the universe's youth.

SGRs, meanwhile, have been linked with magnetars, neutron stars whose extreme magnetic fields slow the star's spin and cause bursts of soft gamma radiation.

Except for a few puzzling hard outbursts.

"They're very different from the other events in terms of their spectra and time history," explained Dr. Pete Woods of the University of Alabama in Huntsville. He works at NASA's Marshall Space Flight Center where he uses data from the center's Burst and Transient Source Experiment (BATSE) aboard the Compton Gamma Ray Observatory as well as from other spacecraft.

Left: Light curves (25-2000 keV) of two bursts from SGR 1900+14 detected with BATSE. The lower graphs show the relative hardness of the events (-2 is "harder" than -1). Links to 905x673-pixel, 135KB JPG. Credit: Pete Woods, University of Alabama in Huntsville and NASA/Marshall. Copies of these graphs, plus the abstract, are available as a 109KB Acrobat PDF.

"SGRs in general show very little or no spectral evolution," Woods said. "These [two flares] were different from normal SGRs and from gamma-ray bursts in that as the burst intensity diminished, the spectrum became harder."

In a paper published today by the Astrophysics Journal Letters, Woods and his colleagues analyzed two bursts from SGR 1900+14 and found that they closely resembled the tough, star-killing gamma-ray bursts observed in deep space, aside from a large difference in peak luminosity.

These events were not exceptionally bright, only hard, a bit longer than typical SGR bursts, and they have evolving spectra.

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"What I find really amazing is the time-integrated spectra of these events match those of GRBs, despite a difference of 100 trillion in peak luminosity!" Woods said.

Most SGR bursts last about 1/10th of a second, but the SGR flares studied by Woods et al. lasted 10 times as long and showed peculiar characteristics. The two flares studied by Woods have spectra similar to the initial pulses of the two giant SGR flares that occurred on March 5, 1979, and Aug. 27, 1998 (see "Happy Birthday Magnetars" and "Crusty Young Star Makes Its Presence Felt" in Web Links below). Woods' flares, though, are about 1,000 times dimmer than the giant events.

Right: When the photon counts per second are plotted against the energy, the resulting graphs for both events more closely resemble one of the principal models for a hard gamma-ray burst than the 200 normal outbursts by the SGR during 1998-99. Links to 895x669-pixel, 153KB JPG. Credit: Pete Woods, University of Alabama in Huntsville and NASA/Marshall.

On Oct. 22, 1998, SGR 1900+14 (the numbers give its position in the night sky) erupted with a 1-second FRED burst, meaning fast rise and exponential decay. It's like a match flaring quickly then dying over a longer period. On Jan. 10, 1999, SGR 1900+14 let loose with a strikingly similar burst.

The total energies of the two bursts, though, are not spectacular, only averaging, about 1.7 to 2.9 x 10⁴⁰ ergs, miniscule compared gamma-ray bursts which are about a trillion times more powerful.

Woods said that several mechanisms could be responsible for the two "hard" SGR bursts.

But the very fact that they happened opens another possibility.

"One question is, How many of these are in the BATSE catalog and have been listed as gamma-ray bursts?" Woods said. "The short answer is not very many. That would skew the observations towards anisotropy, so that supports the belief that these are rare events."

Left: Two strangers sit in the middle of a plot of spectral hardness versus duration. The blue crosses represent 1,191 bursts from the BATSE 4B catalog. The two red asterisks are the burst-like flares that came from SGR 1900+14. Links to 843x709-pixel, 241KB JPG. Credit: Pete Woods, University of Alabama in Huntsville and NASA/Marshall.

Isotropy means that something happens evenly in all directions like sound radiating from a firecracker. Anisotropy means the event is focused or clustered in a direction or area like the firecracker being set off inside a megaphone.

In the case of gamma-ray bursts, the pattern is highly isotropic; bursts appear to be spread randomly across the sky. If more than a handful of SGRs were mistaken for hard gamma-ray bursts, then the BATSE team would see a slight clustering towards the equatorial plane of the galaxy. They have not.

"But there could be a handful in the BATSE database waiting to be found," Woods said. That investigation is under way.

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Outbursts Result in Controversy -- Scientists have different ideas to explain the behavior of Soft Gamma Repeaters (SGRs). (October 20, 1999)
Happy Birthday, Magnetars -- Twenty years since SGR 0526-66 made its grand appearance to Astronomical minds. (March 5, 1999)
Crusty young star makes its presence felt: Gamma ray flash zaps satellites, illuminates Earth, and sheds light on several mysterious stellar events. (Sept. 28, 1998)
A whole lot of shakin' going on: Starquakes lead to discovery of first new Soft Gamma Repeater in 19 years (July 9, 1998)
Magnetar discovery announcement including more details, interviews and more illustrations (May 20, 1998)
Magnetars.org website

The Rossi X-ray Timing Explorer Learning Center at Goddard Space Flight Center

Burst and Transient Source Experiment (BATSE) Home page

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