Mysterious magnetar

Print edition : September 06, 2013

Magnetic loop on magnetar SGR 0418. Photo: ESA

SCIENTISTS using the European Space Agency’s XMM-Newton space telescope have discovered that a curious dead star has all along been hiding one of the strongest magnetic fields in the universe. The object, known as SGR 0418+5729, is a magnetar, a particular kind of neutron star, located in the Milky Way about 6,500 light years from the earth.

A neutron star is the dead core of a once massive star that collapsed in on itself after burning up all its fuel and exploding in a dramatic supernova event. They are extremely dense objects, packing more than the mass of our sun into a sphere only some 20 kilometres across. A small fraction of neutron stars become magnetars, named for their extremely intense magnetic fields, billions to trillions of times greater than those in MRI (magnetic resonance imaging) machines. These fields cause magnetars to erupt sporadically with bursts of high-energy radiation.

SGR 0418 was first detected in June 2009 by space telescopes, including the National Aeronautics and Space Administration’s (NASA) Fermi and Roscosmos’ Koronas-Photon, when it suddenly lit up in X-rays and soft gamma rays.

“Until very recently, all indications were that this magnetar had one of the weakest surface magnetic fields known; at 6 x 10 gauss, it was roughly 100 times lower than for typical magnetars,” says Andrea Tiengo of the Istituto Universitario di Studi Superiori, Pavia, Italy, and the lead author of the work. “However, we suspected that SGR 0418 was in fact hiding a much stronger magnetic field, out of reach of our usual analytical techniques.” The finding has been reported in Nature.

Magnetars spin more slowly than neutron stars, but still complete a rotation within a few seconds. The normal way of determining the magnetic field of a magnetar is to measure the rate at which the spin is declining. Three years of observations had led astronomers to infer a weak magnetic field in SGR 0418.

The new technique developed by Tiengo and his collaborators involves searching for variations in the X-ray spectrum of the magnetar over extremely short time intervals as it rotates. This method allows astronomers to analyse the magnetic field in much more detail and has revealed SGR 0418 as a true magnetic monster. According to him, it has a super-strong, twisted magnetic field reaching 10 gauss across small regions on the surface, spanning only a few hundred metres across.

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