HUNTSVILLE, Alabama -- If you're looking for a growth industry in astrophysics, try magnetars.
The extremely magnetized neutron stars have become the darling of some astronomers, who study them alongside true gamma-ray bursts.
"These are two of the hottest topics in astrophysics today," said Dr. Gerald Fishman of NASA's Marshall Space Flight Center. Fishman is principal investigator for the Burst and Transient Source Experiment, one of the premier instruments for detecting gamma-ray bursts. The experiment flies aboard the Compton Gamma-Ray Observatory, now in Earth orbit.
After lurking in the background at the biennial Huntsville Gamma Ray Burst Symposium, a magnetar workshop leads this years meeting of astronomers working in the field.
Scientists initially thought all gamma-ray bursts had a common cause. But they now know they are dealing with two separate phenomena.
True gamma-ray bursts, which come from the depths of the universe, remained a primary topic at the fifth Huntsville meeting. But Monday belonged to magnetars.
When massive stars go supernova they produce a magnificent nebula. But if the star is not massive enough to produce a black hole, it usually leaves behind a neutron star.
A neutron star crams as much mass as our sun into a sphere just 10 miles across. Squeezing out the empty space that makes up most of the suns volume, neutron stars leave naked atomic nuclei crammed cheek-by-jowl.
This is no mere dull cosmic billiard ball. It rotates, has an intense magnetic field and a thin crust of iron nuclei packed into a crystalline lattice.
In a 1992 theory proposed by Dr. Rob Duncan of the University of Texas at Austin and Dr. Chris Thompson, of the University of North Carolina at Chapel Hill, occasionally a neutron star -- and the causes are still unknown -- has an extra strong magnetic field. At about 44 trillion gauss, the magnetic field is 1,000 times stronger than that of an ordinary neutron star. By comparison, the Earth's magnetic field is a tame 0.6 gauss, and a refrigerator magnet's, a feeble 100 gauss.
"The most a human being can normally expect to be exposed to in his life is about 100,000 gauss from a magnetic resonance imager," Duncan explained. A field of 1 billion gauss "would turn you into magnetized mush."
Duncan and Thompson published their magnetar theory to explain three odd "soft gamma repeaters" (SGRs), gamma ray sources that repeated at irregular intervals and were not gamma-ray bursters. A fourth has since been discovered.
Since the stars magnetic field drags on it, it slows it each rotation. The losses are almost imperceptible --about 1 part in 100 billion.
But that represents a lot of energy since it's braking such a compact yet massive object. In the span of about 10,000 years it slows down to become an Anomalous X-ray Pulsar, an apparent youngster with the gait of a senior citizen. Only six are known.
Under the magnetar theory however, one way that energy is released is when the diamond-like crust suddenly cracks, shifting and pumping energy into the ionized gases trapped around the magnetar.
The result of the starquake arrives at Earth as brilliant gamma-ray flares. On Aug. 27, 1998, such an outburst ionized as much of the Earth's outer atmosphere as the sun would at high noon.
The theory has its challengers.
"We in California know that earthquakes don't last for a fraction of a second," said Dr. Richard Rothschild of the University of California at San Diego. Based on detailed studies of one SGR, he believes that the slowing of the star is caused not by the magnetic field but by its generating a wind that departs at close to the speed of light.
Joining him is Dr. David Marsden of NASA's Goddard Space Flight Center.
"There is evidence that environmental effects may influence development of a magnetar," Marsden said.
He would expect about eight of the 10 candidates to be in "super bubbles," or a volume of empty space carved out by stellar explosions. This is where 80 percent of neutron stars are found. Instead, all 10 magnetar candidates are in areas where the interstellar gas and dust are denser.
And this could be where the interstellar medium hits the fan: Marsden said that the rotating magnetic field could cause a "propeller effect" that flings off material into the interstellar gas and dust at near-light speed velocities, and thus causes gamma-ray flares. The randomness of the outbursts would match the randomness of gas and dust pockets in space.
"It roughly explains the numbers of these objects," Marsden said. Given various factors involved in a neutron star's birth, "Out of 500 young neutron stars, you would expect to see about 10 (as apparent magnetars), which is what we see."
Duncan and Thompson are sticking to their theory and have their defenders. All agree, though, that more observations are needed. The Chandra X-ray Observatory will focus on several candidates, revealing more about them.
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