Where are these stars hiding?
The search for magnetars is not confined to our galaxy. In fact, the first known magnetar burst, which was detected in March 1979, came from the Large Magellanic Cloud, a satellite galaxy to the Milky Way. This finding suggests other extragalactic magnetars exist. A flare within 100 million light-years of Earth could be detected with current X-ray and gamma-ray instruments, provided the flare is as bright as SGR 1806–20’s 2004 event, says University of California, Berkeley, astronomer Kevin Hurley. “Since there are many galaxies within this range, we should see these events frequently,” he notes.
NASA’s Swift satellite, which launched in November 2004 to find gamma-ray bursts, can potentially “open up a new field of astronomy — the study of extragalactic magnetars,” says Duncan. For example, on September 6, 2005, Swift spotted a short-duration burst that might prove to be a magnetar flare emanating from a distant galaxy, although not all scientists accept this interpretation.
Swift and other instruments detected a November 3, 2005, burst from M81, a galaxy about 13 million light-years away. This, says Duncan, was “the first identified extragalactic magnetar flare outside the Local Group.” The flare’s energy release appears comparable to SGR 1806–20’s big blast in 2004.
For astronomers hunting magnetars outside of our galaxy, NASA’s Swift is easily the best instrument around. Still, the satellite isn’t optimized for this task. Swift’s detectors are tuned to lower-energy (spectrally “softer”) events expected from neutron-star mergers. “With the right instruments flying,”
Duncan explains, “we could detect a magnetar flare each week. Swift can’t find that many but may still spot an appreciable number over its lifetime.”
The goal, simply put, is to build up statistics, says Hurley. “We’d like to know the magnetar birth rate, which may be difficult to calculate if we’re limited to the magnetars in our own galaxy.”
In addition to computing the fraction of stars that turn into magnetars, researchers would like to ascertain these stars’ properties. Why does one supernova explosion become a magnetar, while another produces a pulsar? The question extends beyond an interest in magnetars alone. “We’re talking about the endpoint of stellar evolution,” Gaensler explains. “If you want to learn about the star cycle and understand what happens when stars die, you need to understand magnetars.”
Mass, the property that most determines a star’s destiny, is definitely an important part of the equation, but it’s not the whole story. To make a magnetar, the core of the pre-supernova progenitor star must rotate rapidly (about 1,000 times per second) at the end of its life. In Duncan and Thompson’s picture, the core acquires its magnetism through a dynamo effect by converting rotational energy into magnetic energy.
The best current models indicate massive stars rotate more rapidly as they die than when they’re young. To wind up as a magnetar, however, the star has to shed much of its mass before it goes supernova, perhaps by expelling it in strong stellar-wind outflows. Stars with high metal content — elements heavier than helium — have stronger winds, Gaensler notes. So, in addition to looking at the masses of cluster stars, scientists should look at metal content, too. “You wouldn’t expect to find magnetars in a low-metallicity cluster,” he says.
To Thompson, the question of the source of magnetars boils down to this: “What kinds of stars end up with rapidly rotating cores?” That’s difficult to say, he notes, “since no one knows how rotation evolves in the center of a massive star.” And although astronomers can measure how fast a star’s exterior is spinning, they still can’t correlate rotation in the outer layer with what’s going on inside.
Given the uncertainties at the theoretical end, perhaps our best recourse is to see where the observations are taking us, Woods notes. Duncan agrees: “Many details of how magnetars behave are poorly understood. To a great extent, theorists are now being led by the observations.” That’s ironic, because when he and Thompson first dreamed up magnetars, there was little evidence the objects existed.
Thompson, for his part, finds it exhilarating to track the magnetar data now coming in. While the findings have supported some of the early views he and Duncan advanced, they also have served to underscore the many puzzles scientists must still resolve.
Fortunately, growing numbers of astronomers are now interested in taking on these challenges. Indeed, many high-energy astrophysicists regard the December 27, 2004, magnetar burst as a watershed event. It’s comparable in significance to Supernova 1987A, the first naked-eye supernova in centuries and the only one from which neutrinos were detected. Recognition of this has brought more attention to magnetars.
“It’s certainly becoming more of a mainstream field, and we’re attracting talented, new people all the time,” Woods explains. “That has led to some nice new results, and it’s only going to get better.”