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Rare magnetars just beginning to give up their secrets

Elizabeth Howell, News Writer
May 15, 2014, 14:08 UTC

Sen—Rocked by starquakes, and featuring a powerful magnetic field, bizarre objects called magnetars are a rare breed in the universe. Only 21 of these supernova remnants are confirmed after a generation of examining them, according to the Online Magnetar Catalog of McGill University, Canada. 

We are just beginning to understand their nature and their formation. On 14 May, the European Southern Observatory (ESO) announced confirmation of a theory for how the magnetar CXOU J164710.2-455216 formed. It was through the interaction of two closely orbiting massive stars that traded gas, one from the other, until one exploded and threw the other out of its orbit.

Astronomers had thought of this scenario beforehand, but they were unable to find the companion star until recently, when they spotted Westerlund 1-5 escaping its parent star cluster, Westerlund 1. The discovery, made by a team led by Simon Clark of The Open University in the UK, was announced by the European Southern Observatory and will soon appear in the journal Astronomy and Astrophysics.

What is a magnetar, however? It's a rare scenario for the end of a massive star's life. When a supernova happens, typically a star will turn into either a black hole or a neutron star. The black hole happens when gravity in a local area is so immense that it collapses into a singularity. A neutron star, which usually happens when gravity is somewhat less, is a very small object of immense density. An oft-quoted statistic is that a teaspoon of this material would weigh about as much as a mountain.

A simple video guide to magnetars. Credit: Space Scoop

Neutron stars also come in types: pulsars and magnetars. Pulsars are rapidly spinning objects that transmit radiation beams as they spin, providing a sort of celestial clock since their movements are so precise. Magnetars are a more exotic breed, featuring a highly magnetic field and frequent starquakes, among other things.

The new find also explains why the magnetic field is so potent: it's because as the magnetar's origin star picks up mass from the companion star, it spins rapidly, which seems to generate the field. Astronomers will need to make more observations of magnetars to confirm this theory, however.

CXOU J164710.2-455216's class of objects includes other bizarre magnetars. In 2008, astronomers spotted an object (called SWIFT J195509+261406) that flashed 40 times in visible light and then vanished, likely because it was a magnetar whose magnetic field generated the short but strong activity. And in 2013, another team characterized SGR 0418+5729 as having a weaker magnetic field than other magnetars, closer to that of other neutron stars.

Magnetar

Artist's impression of SGR 0501+4516, a magnetar -- an extremely rare object in the cosmos. Image credit: NASA/Goddard Space Flight Center Conceptual Image Lab 

Signs of the first magnetar were spotted in 1979, although it took several more years for astronomers to characterize them. According to Robert Duncan, a University of Texas at Austin who co-proposed the magnetar definition in 1992, astronomers were tipped off that something new was happening when they noticed several incidents thath year of "soft gamma repeaters".

At first these were classified in the same category as bursts of gamma rays (phenomena now believed to happen during supernova collapses), but on a web page written in 2003, Duncan explained the difference. "Unlike GRBs, which have never been verified to come more than once from the same spot in the sky, SGR bursts repeat sporadically from the same source. The gamma rays in SGR bursts are also "spectrally soft" compared to those in GRBs. This means that the average energy per gamma-ray photon is less."

Astronomers now believe the key to a magnetar may be this transfer from one star to another, which might explain why they're so rare. With a sample size of one, however, it would be useful to find more runaway companion stars to confirm the hypothesis.