Outbursts result in controversy

October 19, 1999



Scientists present different explanations for phenomena
October 20, 1999: Just a little over a year ago, scientists announced the discovery of the first known magnetars, neutron stars with exceptionally strong magnetic fields. The announcement seemed to solve two mysteries that had baffled scientists since the 1970s.

But the discovery, and the grouping of ten objects as the first magnetar candidates, has not been universally accepted. As is often the case in science, alternative, plausible explanations are being offered.

"This is one of the hottest topics in astrophysics today," Dr. Jerry Fishman of NASA's Marshall Space Flight Center said at the start of the Fifth Huntsville Gamma Ray Burst Symposium. The symposia are largely devoted to the studies of gamma ray bursts which are believed to come from deep in the universe. Much of the field is driven by the Burst and Transient Source Experiment aboard the Compton Gamma Ray Observatory that has recorded more than 2,500 gamma-ray bursts since April 1991.

Increasingly, instruments aboard other satellites and on the ground have played a crucial role.

Always lurking in the background at past symposia have been discussions of a phenomenon called Soft Gamma Repeaters, or SGRs. Where Gamma Ray Bursts (GRBs) never come located within or quite near our galaxy. Most scientists concluded that they were produced by and identified as neutron stars, the superdense, collapsed remnants of a star's core.

But a 1998 discovery showed that an SGR's rapid slowdown matched what was predicted for magnetars - intensely magnetic neutron stars - that had been proposed in the early 1990s but not confirmed. Part of the magnetar theory held that as SGRs aged, they would become Anomalous X-ray Pulsars (AXPs) mixing characteristics of old and new spinning X-ray stars, and then would fade from sight altogether.

Magnetars live a fast and furious youth and then quickly go out to pasture. The current theory is that for about their first 10,000 years they are Soft Gamma-ray Repeaters. Their burst activity drops sharply and for the next 30,000 year they are Anomalous X-ray Pulsars. All the while, the magnetic field is putting the brakes on the magnetar, slowing its rotation and expending energy through starquakes and magnetic field realignments. After 30,000 to 100,000 years, the AXP is just a dark, spinning neutron star - a "dead" magnetar that is virtually undetectable. Because a magnetar's active phase is so brief, the implication is that the galaxy is filled with millions of dead magnetars.

Magnetars quickly became the subject of intense study and earned a spot with their own workshop at this year's symposium. "The field is very exciting," Fishman said. "It's also very competitive." That became apparent as scientists presented alternative theories for the causes of what is observed in SGRs and AXPs. SGRs emit high-energy X-rays at irregular intervals, perhaps every 20 to 100 years, with immense flares of radiation. As a neutron star, a magnetar has a thin crust of iron nuclei. Beneath that is a bizarre superfluid of neutrons rotating with the star. According to the magnetar theory, giant flare events occur when the neutron crust cracks and readjusts itself, causing starquakes that pump energy into the magnetic field around the star.

Maybe.

"We in California know earthquakes don't last a fraction of a second," said Dr. Richard Rothschild of the University of California at San Diego. They rumble for several seconds and perhaps minutes as compared to the 0.1-second impulse for more than half the outbursts by the SGR known as 1900+14 (the numbers are its coordinates in the sky in right ascension and declination).

Instead of an intense magnetic field being the cause, Rothschild suggests that an exceptionally intense "luminous" wind of interstellar material surrounds the SGR.

"At this time in the life of the object, we believe that the relativistic wind is causing the spindown" of the SGR. The star's period is increasing by about a 10 billionths of a second every second - still more accurate than a very good watch. Under the magnetar theory, the intense magnetic field is responsible for the steady spindown.

"The bottom line is we believe the magnetic field is 6 x 10^13 Gauss," Rothschild said of the field required to do this. By comparison, Earth's magnetic field is a fraction of a Gauss, and the strongest magnetic field a human is likely to encounter is 10^5 Gauss in magnetic resonance imaging at a hospital.

Another proponent of the alternative explanation is Dr. David Marsden of NASA's Goddard Space Flight Center.

"There is evidence that the environment may influence the development" of SGRs, he said. In a "nature vs. nurture" discussion, he noted that the magnetar theory holds that all the properties of the magnetar come from the object itself. "But what if the properties were due to environmental factors?" he asked.

He said that 80 percent of neutron stars form in "superbubbles," regions of space where the interstellar medium is very thin from earlier stellar explosions sweeping the volume clear. Less than 20 percent form in denser areas.

"Then we would expect most of [the magnetars] to reside in the diffuse phase of the interstellar medium," he continued. "In fact, they don't." All are found in the denser areas.

Marsden said the spindown could be explained by the "propeller effect" where the strong magnetic field effectively beats against the interstellar medium as the magnetar moves through space, "and in some cases will eject material into the wind."

The propeller effect could account for the rotational slowdown.

"It roughly explains the numbers of these objects," he concluded. "Out of 500 young neutron stars, you would expect see about 10 [as magnetars], which is what we see" when SGRs and AXPs are counted.

Magnetars were not without their defenders. The magnetar "inventors," Dr. Chris Thompson of the University of North Carolina at Chapel Hill, and Dr. Rob Duncan of the University of Texas at Austin, were present to discuss refinements of their theory. Both maintain that the steady spindown of SGRs is due to star's magnetic field, the most constant factor in its life. Dr. E. V. Gotthelf of Columbia University described yet another candidate for an SGR known as Kes 73, a "surprising discovery." Only four SGRs are confirmed; a fifth flared so briefly in 1997 that its identity is unconfirmed.

Kes 73, observed with Japan's Advanced Satellite for Cosmology and Astronomy (ASCA) reported an 11.8-second period, the longest for such an X-ray pulsar. It's also just 2,500 years old, quite young to be spinning so slow. "In the center we see a very nice point source that dominates the X-rays," he said. ASCA has also shown that Kes 73 is rapidly slowing down, loosing about 4.7x10^-11 second per second. Over the course of ten years, "this seems very steady, remarkably steady, with very little noise. This says it's not an accretion system," one that gobbles material from a companion star. He proposed that Kes 73 was born spinning fast and then spun down fast. The current spindown rate and the age of the remnant match.

"We feel like the Kes 73 pulsar provides the first observational evidence for the magnetar theory," he said. Stepping farther out on a limb, he suggested that magnetars are the rule rather than the exception, that pulsars like the Crab Nebula "are rare anomalies" and radio-quiet AXPs like Kes 73 may account for the large number of missing pulsars in supernova remnants.
-end-


NASA/Marshall Space Flight Center--Space Sciences Laboratory

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