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interview with Dr. Kouveliotou
July
9, 1998: A series of at least 26 starquakes
has led astronomers to the discovery of the first new Soft Gamma
Repeater (SGR) in almost 19 years - and
only the fourth conclusively identified - by scientists at NASA's
Marshall Space Flight Center. The Marshall science team is working
with the Rossi X-ray Timing Explorer team and other scientists
to refine the location and see if the new SGR is associated with
a supernova remnant in the area.
SGRs are neutron stars that emit bursts of soft or low-energy gamma rays at irregular intervals. They are unlike most gamma ray bursts which are one-time events, going off like a cosmic firecracker and then never heard again. In 1986, astrophysicists realized that at least three sets of early gamma ray burst events were repeaters and did not match the "hard" gamma ray energy profile emitted by most bursters. They dubbed this new class the Soft Gamma Repeaters.
Right: A scientist's computer rendering depicts how a magnetar might appear. The thin blue lines are renderings of the superstrong magnetic field lines of this kind of star. Click the image for a 1024x1024 pixel, 77KB jpeg image. For best quality, a 1.6MB targa file is available.Credit Dr. Robert Mallozzi/University of Alabama in Huntsville, and Marshall Space Flight Center.
SGRs are believed to be just one short phase in the life of a magnetar, a neutron star with an extremely powerful magnetic field. If the magnetar theory is correct, and SGRs are indeed the early phase of magnetars, then SGR outbursts are caused by massive starquakes as the magnetic field wrinkles the star's crust. These wrinkles are only a few millimeters high, but release more energy than all of the earthquakes that the Earth has experienced.
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Above, right: A graph depicts the history of SGRs, including the periods when spacecraft were available to observe them. (Links to 712x566-pixel, 84KB JPG; click here for a 150KB PDF image in higher resolution). Credit: NASA/Marshall Space Flight Center.
"There's no doubt about this one," said Dr. Chryssa Kouveliotou, a scientist with the Universities Space Research Association who works at NASA/Marshall. "It went off several times and was very, very powerful. This is quite exciting." |
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The new SGR triggered BATSE 26 times during June 15-22, including an incredible 12 bursts on June 18. Each burst lasted 2/10ths of a second, typical for an SGR. A preliminary analysis of the bursts indicates that their energy spectra are softer than what comes from super-energetic gamma-ray bursts that go off deep in the universe (that is, the photons are in the part of the spectrum where X-rays become gamma rays). Another five bursts from the same area of the sky were recorded on June 17 and 18. The last burst peaked at almost 500,000 counts per second, making for a powerful source.
BATSE
was designed with eight detectors that cover the entire sky. The
location of a burst is determined by how brightly each of the
eight faces sees the burst.
Right: A sky chart depicts the location of SGR 1627-41 as defined by BATSE (white area; BATSE saw it on two occasions in which the Earth's limb partially blocked the sky, indicated by the gray areas), elements of Interplanetary Network (blue band), and the Rossi X-ray Timing Explorer (yellow circles). SGR 1627-41 is on the plane of the Milky Way galaxy and appears to be very close to a known supernova remnant, G 337.0-0.1. (links to 600x600-pixel, 29KB GIF). Credit: NASA/Marshall Space Flight Center.
This new object does not correspond to any known SGR, and has been designated SGR 1627-41 (the numbers indicate its position in the sky). The location has been narrowed by Dr. Kevin Hurley of the University of California at Berkeley by combining data from detectors on several spacecraft, including Hurley's own burst detector on the Ulysses probe in deep space. This Interplanetary Network (IPN) uses the differences in the arrival times of the burst photons at different spacecraft to estimate the location of the source.
The BATSE team noted that the IPN arc passes right through a supernova remnant on the galactic plane, thus indicating a strong correlation between the two.
Kouveliotou has requested that other spacecraft
be repointed to try to catch the fading embers from the burst,
and determine whether the SGR and supernova remnant are linked.
Left: SGR 1627-41 is located roughly in the center of the blue box superimposed on this image of our galaxy. The image was built from data taken by the Cosmic Background Explorer. (links to 1,024x512-pixel, 175KB JPG.) Credit: NASA.
SGRs have mystified scientists since they were first seen 19 years ago. In May, Kouveliotou led a team that concluded that one SGR is a magnetar with a magnetic field about 800 trillion times stronger than that of Earth.
Current theory holds that following a supernova, a lone, super-magnetized neutron star will appear as an SGR for about 10,000 years, then become a weaker character called an Anomalous X-ray Pulsar for another 10,000 years or so, then fade below the limits of detection altogether.
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Credit Dr, Robert Mallozzi/University
of Alabama in Huntsville and Animated Gif movies available (shows a short segment of burst animation; realvideo player not required) |
Dr. Kouvelioutou discusses how starquakes lead to the soft gamma ray bursts that we can see with spacecraft, the discovery of this newest SGR, and how we can detect similarities between starquakes and Earthquakes. Credit NASA/Marshall Space Flight Center |
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These movies require Realvideo player (version 5). Click to download the RealVideo Player (from RealNetworks, Inc. |
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Author: Dave
Dooling
Curator: Linda Porter
NASA Official: Gregory
S. Wilson
In mid-June, though, the Burst and Transient
Source Experiment (BATSE) aboard the Compton Gamma Ray Observatory
(left) registered a series of bursts that clearly came from an
SGR. The bursts were also detected by the All-Sky Monitor aboard
the Rossi X-ray Timing Explorer which can see finer details in
the timing of a burst.
