Spin seen for soft gamma-ray repeater

BY KULVINDER SINGH CHADHA

ASTRONOMY NOW

Posted: 19 January, 2009

For the first time ever, the spin rate of an elusive soft gamma-ray repeater has been detected, which could allow scientists to infer more of its properties. The reason that this is so important is because only five of these objects are currently known; four in the Milky Way, and one in the Large Magellanic Cloud, making for an incredibly small sample to study. The period for SGR 1627-41, the object in question, was inferred from the glow detected by ESA’s XMM-Newton X-ray telescope.

Calculations reveal that SGR 1627-41 has a rotation rate of once every 2.6 seconds. "This makes it the second fastest rotating magnetar known," says Sandro Mereghetti of the INAF/Istituto di Astrofisica Spaziale e Fisica Cosmica in Milan.

Soft gamma-ray repeaters are a highly magnetic form of neutron star, with typical magnetic field strengths 1,000 times greater than for standard neutron stars. A neutron star is the highly dense, collapsed core of a large star that has self-destructed in a supernova explosion. They are typically between ten and thirty kilometers across, yet contains about twice the mass of the Sun.

NASA’s Compton Gamma Ray Observatory discovered SGR 1627-41 in 1998, when it came to life, emitting around a hundred short period flares during a six-week period. But before telescopes could be scrambled to measure its rotation rate, it faded away again just as quickly. Because of this, SGR 1627-41 remained the only magnetar with an unknown period.

XMM-Newton's false-colour X-ray image of the soft gamma-ray repeater (or magnetar) in question. The red area shows the supernova remnant. Image: ESA/XMM-Newton/EPIC (P. Esposito et al).

However, in the summer of 2008, SGR 1627-41 repeated its dazzling display. There was a big problem though: it was located in a region of sky that XMM-Newton couldn’t point to for another four months, as this would have turned its solar panels away from the Sun. So began a tense waiting game during which time SGR 1627-41 began fading fast. When finally XMM-Newton swung round to it in September 2008, the sensitivity of the facility’s instruments meant it was still detectable.

Why magnetars have such colossal magnetic fields is still something of a mystery, but one idea is that they are formed with an initial rapid spin, (two to three thousandths of a second) whereas ordinary neutron stars are formed spinning at least ten times slower. The magnetar’s high-spin rate, and interior convection processes, makes it an efficient dynamo, allowing it to build up an enormous magnetic field.

But with its rotation rate of 2.6 seconds, SGR 1627-41 must have slowed down. Therefore it must old. The magnetar is surrounded by a supernova remnant, from which possibly it formed. XMM-Newton detected the X-ray glow from this debris and Mereghetti says, "These usually fade to invisibility after a few tens of thousand years. The fact that we still see this one means it is probably only a few thousand years old."

The team who conducted the study are eagerly waiting for SGR 1627-41 to flare again. If it does so they will re-measure its rotation rate. Any difference will tell them how quickly the object is decelerating. Finally Mereghetti adds, "These are intriguing objects and we have much still to learn about them."

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