Magnetars: A missing link

Astronomers have found evidence suggesting that a rare group of ultradense stars are magnetars–the objects with the strongest magnetic fields known in the universe.

MAGNETIC INTENSITY. Artist’s view of a magnetar–a rare, ultradense star. Robert Mallozzi, Univ. of Alabama, Huntsville, and NASA Marshall Space Flight Center

According to theory, these enormous fields–100 trillion to 1 quadrillion gauss–are generated within ultradense neutron stars. A neutron star’s gravity is so strong that it squeezes protons and electrons into a fluidlike core of neutrons inside a solid crust. Swirling motions within the core may dramatically amplify the already strong magnetic field there, creating a magnetar.

The field creates unbearable stresses that can be relieved only by cracks in the star’s crust. As the magnetic energy pours out, it powers electrons just above the crust to emit gamma rays. This process of stress buildup, crust cracking, and gamma-ray emissions is cyclic.

Astronomers had identified five neutron stars, known as soft gamma-ray repeaters, as magnetars (SN: 9/12/98, p. 164: http://www.sciencenews.org/sn_arc98/9_12_98/Fob1.htm). Another group of neutron stars, called anomalous X-ray pulsars, has also been considered possible magnetars.

However, these stars had never been observed to emit outbursts of gamma rays.

Until now. Using NASA’s Rossi X-ray Timing Explorer satellite, Fotis P. Gavrill of McGill University in Montreal and his colleagues detected bursts from an anomalous X-ray pulsar called 1E1048.1-5937 in the constellation Carina. Gavrill, Victoria M. Kaspi of McGill, and Peter M. Woods of the National Space Science and Technology Center in Huntsville, Ala., describe their study in the Sept. 12 Nature.

“This discovery at last establishes a strong link between anomalous X-ray pulsars and soft–gamma-ray repeaters,” with both being magnetars, comments Shri Kulkarni of the California Institute of Technology in Pasadena.

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