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Stanford Report, January 5, 2000

Neutron star’s blast 10 times more powerful than believed


A distant star that blasted Earth with a 5-minute flare of gamma and X-rays in August 1998 turns out to have been almost 10 times more powerful than previously thought, say researchers at Stanford and the University of California-Berkeley.

The neutron star, known as Soft Gamma Repeater 1900+14, already held the record for producing the most powerful blast of X-rays measured at Earth from beyond the solar system. "We're now saying that the effect we saw on the planet Earth couldn't have been this great unless the energy released in this event was about 10 times stronger" than previously estimated, said Umran Inan, an electrical engineering professor in Stanford's Space, Telecommunications and Radioscience Laboratory (STAR lab).

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The star source of the flare, estimated to have been some 23,000 light years away from Earth, is believed to have been the collapsed neutron core of an exploded star, one that scientists call a magnetar, because it is believed to have a magnetic field 100 times greater than other known objects, such as a radiopulsar, which has a hundred thousand billion Gauss. (In comparison, Earth's magnetic field has about one-half Gauss.)

This particular star has been releasing a constant X-ray flow from its surface, with occasional starquakes of gamma rays, including the cataclysmic flare that bathed the Earth's atmosphere on Aug. 27, 1998. Astrophysicists, using measurements taken by the spacecraft Ulysses, were the first to report that an invisible but intense flare of energetic photons passed through our solar system. Monitoring temporal variations of the lower ionosphere with very low frequency radio waves at ground stations in the United States and the Antarctic, Stanford researchers soon after reported very large changes in the amplitude and phase of their radio signals, indicating intense enhanced ionization associated with the flare. They said that the magnitude of the changes was such that the ionization levels in the altitude range of 60 to 90 kilometers switched from normal nighttime to daytime levels.

Since then, astrophysicist Kevin Hurley of Berkeley's Space Sciences Laboratory and engineers in Inan's lab put their space and ground data together, adding simulation models to the mix, and reached the conclusion that Soft Gamma Repeater 1900+14 produced between 9 and 10 times more energy than was inferred on the basis of data from the Ulysses instruments. The reason for the discrepancy is that the low energy component of the flare was outside the range of the spacecraft's instrumentation, they wrote in the Nov. 15 issue of Geophysical Research Letters. The total energy released in this event at the source star is now believed to be equivalent to the total energy put out by our sun in 3,000 years. (For the technically minded, this is 4 times 10 to the 44th ergs rather than 4 times 10 to the 43rd ergs, as initially estimated.)

Hurley put it this way: "There's enough energy in this giant flare to satisfy the human energy consumption on earth for four times 10 to the 17th years, or 40 million times the age of the universe."

The finding "is exciting to astrophysicists," Inan said, "because they can now use this work to make new inferences about strengths of stars."

The new result also underscores the need for spacecraft instruments designed to measure low energy gamma rays, although the background levels at these energies is high, making it difficult to measure signals, he said. "It may well be that the planet Earth is our best sensor for these low energy gamma rays."

Co-authors of the study with Inan and Hurley were Inan's graduate students Nikolai Lehtinen, Sean Lev-Tov and Mike Johnson, and senior research associate Tim Bell. SR