Braking glitch may point to massive starquake
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"Braking glitch" may point to massive starquake Soft gamma-ray burst signaled major
change on neutron star
July 19, 1999: If you travel across California, you can find spots where a road or fence is offset several feet to one side, then resumes its path. For most people the reaction is, "Aha! An earthquake has struck."
Scientists are having a similar reaction as they puzzle over an offset or "braking glitch" in the spin rate of a soft gamma repeater or SGR. A graph of the star's spin rate shows a steady increase in its rotational period, but with a break in the line that may have been caused by a massive starquake. The difference only 1 millisecond, a massive difference for an object that packs as much mass as our sun into a ball only 20 km (12 mi) across.
Left: Data from the Compton Gamma Ray Observatory (top) and three X-ray astronomy satellites (bottom) indicate that a break in SGR 1900+14's spin rate may be closely related to a massive gamma-ray outburst. Links to 559x575-pixel, 78K JPG. (Credit: Pete Woods, et al)
"The spindown was nearly constant until the summer," said Pete Woods, an astrophysics graduate student at NASA's Marshall Space Flight Center. "You can't easily connect these two lines. But sometime in the summer of 1998, the star spun down very rapidly, about twice as fast as normal."
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 go off like a cosmic firecracker and are never heard again. In 1986, astrophysicists dubbed this new class the Soft Gamma Repeaters. Only four have been confirmed to exist.
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The magnetar theory was developed by Dr. Chris Thompson of the University of North Carolina and Dr. Rob Duncan of the University of Texas in Austin. Woods says that much of his interpretation of the Aug. 27 event is based on their theory. Working with Woods on the Aug. 27 event, Thompson and Duncan developed a physical interpretation for the "braking glitch."
If the magnetar theory is correct, 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 ever experienced.
That probably is what happened with SGR 1900+14 on Aug. 27, 1998. The star's gamma-ray bursts are readily detected by the Burst and Transient Source Experiment (BATSE) aboard the Compton Gamma Ray Observatory (although it was not in position for the Aug. 27 event). It was designed as an all-sky monitor, not to see the fine timing structure of a burst. Fine-timing of the pulsars is studied by other satellites, most notably the Rossi X-ray Timing Explorer, plus Japan's Advanced Satellite for Cosmology and Astrophysics (ASCA) and the Italian-Dutch Beppo SAX.
SGR outbursts come in two forms. Most are starquakes that occur when the kilometer-thick metallic crust shifts and pumps energy into the plasmasphere around the neutron star. Such an event is depicted here. The really big bursts, like the March 5, 1979, event, may be caused by massive readjustments of the magnetic field. (Images created by Dr. Robert Mallozzi, University of Alabama in Huntsville)
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Then, on Aug. 27, 1998, it erupted with a 6-minute-long burst about 1,000 times more powerful than any single event in its history. It has since settled down, but continues to rumble.
Within a day of the Aug. 27 eruption, Rossi was repointed at SGR 1900+14 to measure the rotational rate. Beppo SAX and ASCA followed in 18 and 19 days, respectively. Together, they provide a striking "before-and-after" portrait of SGR 1900+14.
Making a neutron star - and a magnetar - starts (1) with a massive star that has burned up all of its fuel, then (2) collapses and causes a massive explosion, the supernova. that blows off the outer layers and (3) compresses the core. Soon, all that is left is a shell of expanding gas (not always this pretty or symmetrical) and a rapidly spinning neutron star at "ground zero." If the original star was spinning fast enough and had a strong enough magnetic field, the neutron star is a magnetar.
As far back as September 1996, data from Rossi showed SGR 1900+14 had a period of 5.1558157 seconds. By March 1999, the period had increased to 5.16156 seconds, a difference of about 0.006 second, almost 1/200th of a second, indicating the rotation of the star had slowed.
With the 80-day gap in timing observations - "we can't predict when a burst will happen," Woods noted - astronomers have two scenarios for the slowdown: it happened gradually, or it happened all at once.
"It seems logical to attribute the slowdown to the Aug. 27 burst," Woods said, because a gradual slowdown with little gamma-ray activity then punctuated by a burst at the end does not make sense.
Woods said that it all may be tied up with the strange nature of neutron stars, the only stars (along with some dwarf stars) that have a solid surface. They are formed when a supernova explosion compresses a star's core. The neutron star has a tough, diamond-like crust, about 1 km (0.6 mi) thick with part of the dead star's magnetic field frozen into the crust. Beneath the crust is a deep, roiling ocean of neutrons. It's a strange, intense mimic of the Earth's continents gently being pushed by convection in the mantle.
While magnetars can undergo the same sort of glitches as radio pulsars, the Aug. 27 event apparently worked in reverse. Like a radio pulsar, the magnetar has a rigid crust and superfluid interior. It also has a trapped magnetic field that is more than 100 times stronger than those of ordinary pulsars. It's so intense that the magnetic field acts as a brake, causing the gradual spindown.
"The crust and interior are loosely coupled," Woods continued. "But if you do something drastic, rotational energy may be exchanged between the two."
That something drastic may have been a magnetic field which constantly restructures the neutron star crust, Woods continued. A gradual reshaping or motion of the crust can twist or move the vortices of flowing neutrons under the surface. In addition, a sudden fracture will dislocate the vortices and transfer angular momentum between the fluid and the crust.
The transfer can spin up or - in this case - spin down the neutron star.The exact physics of what is going on inside the neutron star is poorly understood.
"We're still learning," Woods said. "It's not that well known." That's because no one is sure just what kind of materials they are dealing with. The densities and temperatures are beyond anything that can be simulated on Earth, even in a computer.
"Trying to understand what's going on is very difficult," Woods added. RXTE continues to observe both SGR 1900+14 and SGR 1806-20 to build up their spin histories.
"This will allow us to study, in greater detail, how these stars spin down," Woods said.
- Burst and Transient Source Experiment (BATSE)
- Rossi X-Ray Timing Explorer
- The mysterious gamma ray bursts that still puzzle astronomers
- BATSE.COM web site
- NASA's Office of Space Sciencepress releases and other news related to NASA and astrophysics
- Energy burst from star disturbed Earth's environment - Dr. Umran Inan expands on the local effects of the Aug. 27 flare.
- Magnetar shows off in radio frequencies, too - Announcement by the National Radio Astronomy Observatory.
- SGR and Magnetar web pages (University of Texas, Dr. Robert Duncan)
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