Deconstructing Magnetars

29 May 2008 (All day)


Mystery disk.
NASA's Spitzer Space Telescope spied this thin, glowing dust ring around a dead star called a magnetar--unseen in this photo.

Tiny, dense, and turbulent, a magnetar is one of the most mysterious objects in the universe. Now, two studies provide clues about how these short-lived and puzzling stars form and add credence to the idea that when magnetars collide, they might create equally mysterious phenomena: gravity waves.

When a giant star explodes, the burned-out remnant sometimes becomes a magnetar. These objects contain more mass than the Sun and boast a magnetic field trillions of times more powerful. Instead of surviving for tens of millions of years, like their massive progenitors, magnetars fizzle out in less than 100,000 years, after which their sometimes violent magnetic activity--along with nearly all of their radiation release--ceases, rendering them all but undetectable. For this reason, astronomers suspect that the Milky Way might be littered with dead magnetars.

One of the new studies could help clear up some of the many enigmas about magnetars, such as why they form and why they don't behave like their more abundant cousins, called neutron stars, whose magnetic fields are much weaker. A U.S.-British team has found a ring around a magnetar like nothing ever seen before. Surrounding a magnetar called SGR 1900+14, which was discovered in 1986, the ring is so thin it's almost two-dimensional, and it emits no radiation other than a faint, infrared glow. Reporting today in Nature, the team says the ring probably formed after the magnetic star emitted a giant flare, spotted in 1998, which incinerated surrounding dust in all directions, leaving only the thin disk. The disk glows from the heat emitted by nearby massive stars, which the team thinks are relatives of the magnetar's forebearer. The researchers say they hope to nail the original mass of SGR 1900+14 by determining the masses of those relatives, and the resulting data could help them work out how heavy a star needs to be to become a magnetar rather than a neutron star.

Meanwhile, a U.S.-German team has created a supercomputer simulation of two magnetars colliding and found that the stars' magnetic fields exert more influence than previously expected on the products of the collision. Although this point might seem esoteric, scientists have been searching for decades for ripples in spacetime called gravity waves. The waves remain undetected so far, but they are predicted by Einstein's General Theory of Relativity and therefore remain a key ingredient in the makeup of spacetime.

Previous simulations had not checked how much stellar magnetic fields affect gravity waves originating from neutron-star collisions. But the team reported 15 May in Physical Review Letters that such fields do determine how the colliding stars are ripped apart just before they collide and how their crushing gravity slams them back together. If confirmed, the result suggests that magnetar collisions could be a common source of gravity waves.

Determining the properties of magnetar progenitors will answer important astrophysical questions, says astrophysicist Fotis Gavriil of NASA's Goddard Space Flight Center in Greenbelt, Maryland. "They're probably more massive than the stars that give rise to ordinary neutron stars," he says," and possibly even as massive as stars thought to spawn only black holes.

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