18 February 2005
Magnetar flare blitzed Earth Dec. 27, could solve cosmic mysteries

This information is co-released with The University of California, Berkeley, and co-incides with a NASA Space Science Update (right).

Austin, Texas — Astronomers around the world recorded late last year a powerful explosion of high-energy X-rays and gamma rays — a split-second flash from the other side of our galaxy that was strong enough to affect the Earth's atmosphere. The flash, called a soft gamma repeater flare, reached Earth on Dec. 27 and was detected by at least 15 satellites and spacecraft between Earth and Saturn, swamping most of their detectors.

Thought to be a mighty cataclysm in a super-dense, highly magnetized star called a magnetar, it emitted as much energy in two-tenths of a second as the sun gives off in 250,000 years. Robert C. Duncan of the University of Texas at Austin originally proposed and developed the magnetar theory, along with Christopher Thompson of the Canadian Institute of Theoretical Astrophysics.

"This is a key event for understanding magnetars,” Duncan said. Its intrinsic power was a thousand times greater than the power of all other stars in the galaxy put together, and at least 100 times the power of any previous magnetar outburst in our galaxy. It was ten thousand times brighter than the brightest supernova.

Duncan and Thompson worked with Kevin Hurley, a research physicist at UC Berkeley who leads a major international team studying the event, to understand the immense power of the Dec. 27 flare. “It was the mother of all magnetic flares – a true monster,” Hurley said.

The team's observations and analysis are summarized in a paper that has been submitted for publication in the journal Nature.

“Soft gamma repeater” bursts — pinpoint flashes of highly energetic X-rays and low-energy (soft) gamma rays coming repeatedly from one place in the sky — were first noticed in 1979 and remained a mystery until Duncan and Thompson proposed in 1992 that they originate from magnetically powered neutron stars, or magnetars. Formed by the collapsing core of a star throwing off its outer layers in a supernova explosion, neutron stars are extremely dense, with more mass than in the Sun packed into a ball about 10 miles across. Many neutron stars spin rapidly. These spinning neutron stars, some rotating a thousand times a second, signal their presence by the emission of pulsed radio waves, and are called pulsars.

According to Duncan, magnetars are a special kind of neutron star. They are born rotating very quickly, which causes their magnetic fields to get amplified. But after a few thousand years, their intense magnetic field slows their spin to a more moderate period of one rotation every few seconds. The magnetic fields both inside and outside the star twist, however, and according to the theory these intense fields can stress and move the crust much like shearing along the San Andreas Fault. These magnetic fields are a quadrillion — a million billion — times stronger than the field that deflects compass needles at the Earth’s surface.

The shear moves the crust around and the magnetic fields are tied to the crust, generating twists in the magnetic field that can sometimes break and reconnect in a process that sends trapped positrons and electrons flying out from the star, annihilating each other in a gigantic explosion of hard gamma rays.

The flare observed Dec. 27 originated about 50,000 light years away in the constellation Sagittarius, which means that the magnetar sits directly opposite the center of our galaxy from the Earth in the disk of the Milky Way Galaxy.

As the radiation stormed through our solar system, it blitzed at least 15 spacecraft, knocking their instruments off-scale whether or not they were pointing in the magnetar's direction. One Russian satellite, Coronas-F, detected gamma rays that had bounced off the Moon.

The flare also ripped atoms apart, ionizing them, in much of the Earth’s ionosphere for five minutes, to a deeper level than even the biggest solar flares do, an effect noticed via its effect on long-wavelength radio communications. Such events are unlikely to pose a danger to the Earth because the chances that one would be close enough to the Earth to cause serious disruption are exceedingly small.

Hurley and his team combined information from many spacecraft, including neutron and gamma-ray detectors aboard Mars Odyssey and many near-Earth satellites, in order to localize it to a spot well-known to astronomers: a magnetar known as SGR 1806-20. This position was accurately confirmed by radio astronomers at the Very Large Array in Socorro, N.M., who studied the fading radio afterglow of the event and obtained important information about the explosion.

The tremendous power of the event has suggested a novel solution to a long-standing mystery — the origins of a strange phenomenon known as “Short-Duration Gamma Ray Bursts.” Hundreds of brief, mysterious flashes of high-energy radiation from deepest space, lasting less than two seconds, have been measured and recorded over decades, but nobody knew what they were.

The similarity between the Dec. 27 burst and these short-duration bursts lies in the brief spike of hard gamma rays that arrives first and carries almost all the energy. In the recent burst, for example, the hard spike lasted only two-tenths of a second. This was followed by a “tail” of X-rays that lasted over six minutes. As the tail faded, its brightness oscillated on a 7.56 second cycle, the known rotation period of the magnetar.

According to Duncan and Thompson’s theory, the oscillating X-ray tail that followed was due to a residue of electrons, positrons and gamma-rays trapped in the magnetar’s magnetic field. Such a hot “trapped fireball” shrinks and evaporates over minutes, as electrons and positrons annihilate. The measurements of Hurley’s team corroborate this picture. The tail’s brightness appears to oscillate because the fireball is stuck to the surface of the rotating star by the magnetic field, so it rotates with the star like a lighthouse beacon.

Duncan and his team argue that the hard initial spike of these giant flares is so bright that it can be detected from very far away, meaning that some of the short flares we see are from other galaxies, though the soft X-ray tails are too faint to be seen.

Duncan and his collaborators predict that if a magnetar flares as brightly as the December 27 event within 100 million light-years of Earth, astronomers should be able to detect it. Texas astronomers John Scalo and Sheila Kannappan helped Duncan estimate the rate at which such distant flares might be seen. They estimated that of order 40% of the short bursts previously observed could have been such magnetar bursts. There is a good probability that the newly-launched Swift satellite will see a magnetar burst once a month.

Launched in November 2004 and gathering data only since January, Swift is designed to automatically turn its X-ray telescope toward a burst in order to accurately pin down its position.

Duncan’s team estimates that Swift will spot an abundance of magnetars lurking in other galaxies. In some cases, Swift’s X-ray telescope may even catch the oscillating tail and measure the rotation period of the faraway star.

“Swift will open up a new field of astronomy: the study of extragalactic magnetars,” Duncan said.

Co-authors with Hurley, Boggs, Duncan and Thompson were D. M. Smith of the UC Santa Cruz physics department, RHESSI and Wind principal investigator and Space Sciences Laboratory Director Robert Lin, and teams of U.S., Swiss, Russian, and German scientists.

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Notes to editors: Robert Duncan and Kevin Hurley will be at NASA Headquarters in Washington, D.C., on Friday, Feb. 18, to attend a NASA Science Update about the Dec. 27 giant flare and observations by the recently launched Swift satellite. Duncan's cell phone number is (512) 587-0043. Hurley’s cell phone number is (510) 366-4463.

Duncan normally can be reached at (512) 471-7426 or at duncan@astro.as.utexas.edu. Hurley can be reached at his office, (510) 643-9173, or via e-mail at khurley@ssl.berkeley.edu. Steven Boggs is at (510) 643-4129 or boggs@ssl.berkeley.edu.

Robert Sanders, science press officer for UC Berkeley, can be reached at (510) 643-6998 or rsanders@berkeley.edu.