Crouching Tiger Hidden Magnetar

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Magnetars are a rare type of neutron star, with powerful magnetic fields making them prone to occasional violent outbursts. Only a small handful of these curious beasts have been found in our galaxy, but new research from the Chandra X-ray Observatory implies that they may be a lot more common than previously expected. They may simply be in hiding.

Magnetars are traditionally thought to show intense magnetic fields on their surfaces, reaching thousands of times the strength of the fields found on regular neutron stars. But 6,500 light years from Earth, one magnetar in particular, SGR 0418+5729, seems to buck the trend. On the surface, it appears to be just an ordinary neutron star.

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It seems, there may be a lot which we don’t know about these massive stellar magnets. Nanda Rea at the Barcelona Institute of Space Science explained that ”we have found that SGR 0418 has a much lower surface magnetic field than any other magnetar,” elaborating that there may be some important consequences for our understanding of both neutron stars and the supernova explosions which create them.

For over three years, researchers kept a watchful eye on SGR 0418, using some of the world’s best x-ray observatories. By measuring changes in its rotation during x-ray outbursts, they managed to accurately estimate the external magnetic field strength of the neutron star.

Strangely, at least on the outside, that magnetic field appeared a lot weaker than they were expecting. And it’s very likely that there are other neutron stars out there which are hiding their true colors. “We think that about once a year in every galaxy a quiet neutron star should turn on with magnetar-like outbursts, according to our model for SGR 0418,” commented José Pons of the University of Alicante, Spain, who hopes that many more such objects may come to light with further research.

Starquakes

In 2004 (on the day before my birthday, as it happens) a blast of radiation saturated every gamma ray observatory in orbit, as well as disrupting Earth’s ionosphere. The source was traced back to a magnetar outburst.

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Because they’re so magnetized, stress accumulates below the surface of these strange objects. Every so often, this stress becomes too much for the crust of the neutron star to take, and — crack! The crust ruptures, causing a starquake.

Neutron stars are weird, wonderful, and extreme objects. The remnant cores of dead stars, they’re typically at least twice as massive as our entire solar system — with all of that mass packed into an object with a radius of just 10 miles. To give some idea of exactly how powerful the forces behind starquakes are, the crust on a neutron star is expected to be about 10 billion times stronger than steel!

When these crust fractures happen, they send out a blast of high energy x-rays and gamma rays. Known as megaflares, these starquakes are responsible for some of the mysterious gamma ray bursts observed throughout the Universe.

Stellar Post Mortem

To gain a better idea of what they were looking at, researchers made a model of how neutron stars cool with age, as well as the way their magnetic fields decay. They estimated that SGR 0418 is over half a million years old.

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This would make it older than most other magnetars, giving ample time for its surface magnetic field to weaken. On the other hand, the magnetic field inside it is still quite strong, meaning that the potential for starquakes is quite literally hiding just beneath its surface. It also suggests that its magnetic field should have been very strong when it was first created. This hints that there may be many many similar neutron stars out there, in hiding.

Where exactly these magnetic fields originated, however, is still an unanswered question. Perhaps the stars which exploded as supernovae had strong magnetic fields themselves, which were passed on to the neutron stars that they left behind. Or maybe these neutron stars were spinning rapidly in the midst of the supernovae which created them, generating fields of their own.

If the research on SGR 0418 is right then there may be as many as ten times the number of magnetars out there than we’d previously thought. It’s entirely possible a large number of the gamma ray bursts we’ve seen could be caused by the birth of magnetars rather than black holes. Their dramatic births could also be picked up quite frequently as gravitational waves — ripples in the fabric of space-time.

What all of this means is that there’s a good chance that our galaxy may contain a large number of elderly magnetars, hiding their magnetic fields from view. Just quietly minding their own business out there in our galaxy. And we might be none the wiser until one of them decided to throw a temper tantrum and spit out a burst of gamma rays.

For those who may be curious, the research paper for this work is available online through arXiv.

Image: A composite image showing SGR 0418 in x-rays and infrared, with a superimposed illustration depicting its magnetic field. Credit: X-ray: NASA/CXC/CSIC-IEEC/N.Rea et al; Optical: Isaac Newton Group of Telescopes, La Palma/WHT; Infrared: NASA/JPL-Caltech; Illustration: NASA/CXC/M.Weiss