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Magnetic Quakes Shake Neutron Stars

20 May 1998 6:00 pm
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Neutron stars, superdense balls of neutrons that already rank among the most extreme objects in the universe, just got weirder. The strongest magnetic field ever detected may encase one such star and literally fracture its surface, scientists report in tomorrow's issue of Nature. These bizarre "crustquakes" appear to fling waves of gamma rays across the Milky Way.

When giant stars explode in the titanic blasts called supernovae, their cores collapse into ultradense cinders. The most massive stellar cores become black holes, but usually they turn into pulsars--whirling neutron stars about 10 kilometers wide that emit pulses of radio waves. Since 1979, astronomers have found several striking exceptions. These neutron stars spew x-rays instead of radio waves, and they unleash irregular bursts of low-energy gamma rays. Some theorists suggested that these unruly objects might be "magnetars"--neutron stars girdled by an intense magnetic field, which crushes the stars' crust once they begin to cool.

New research with the Rossi X-ray Timing Explorer supports that scenario. The satellite detected oscillating bursts of x-rays from a neutron star 40,000 light-years away during one of the star's outbursts in late 1996. The oscillations indicate that the star spins just once every 7.5 seconds--far slower than other young pulsars, which spin at least 10 times per second. To have slowed down so fast, the star must be feeling the drag of a magnetic field 100 times stronger than those around typical neutron stars, says astrophysicist Chryssa Kouveliotou of NASA's Marshall Space Flight Center in Huntsville, Alabama.

Such a powerful magnetic field can crack the brittle surface of the cooling neutron star and send shock waves racing into space, says astrophysicist Christopher Thompson of the University of North Carolina, Chapel Hill, releasing a "trapped fireball" of x-rays and gamma rays. But the display wouldn't last long. When the star grinds to a near halt after a few thousand years, its outbursts should fade, because a slow-moving magnetic field cannot accelerate charged particles to high energies, Kouveliotou says. "There must be millions of these in the galaxy, dead and invisible to us," she notes.

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