Magnetars may fuel briefer bursts

Rapidly spinning neutron stars could be source of some short gamma-ray explosions, astronomers propose

ANNAPOLIS, Md. — It takes a powerful beast to unleash a gamma-ray burst, the most energetic type of explosion since the Big Bang. But the exact nature of the cosmic powerhouse that generates the various kinds of bursts has been a matter of debate for nearly 20 years.

A new finding reported November 3 at the Gamma Ray Burst 2010 meeting suggests that rapidly spinning magnetars, which have the strongest known magnetic fields in the universe, may be the driving force behind a larger population of gamma-ray bursts than scientists had thought.

BURSTING OUT A rapidly rotating, highly magnetized compact star called a magnetar may power a larger population of gamma-ray bursts than previously thought. NASA/Swift/Sonoma State University/A. Simonnet

Magnetars that last several minutes before they collapse under their own weight to form black holes had previously been invoked to explain the formation of some long gamma-ray bursts, energetic flashes of radiation that endure for more than two seconds. Calculations by Paul O’Brien and Antonia Rowlinson of the University of Leicester in England and their colleagues now show that a magnetar could also account for a short gamma-ray burst, specifically a 36-millisecond flash observed on May 15, 2009, by NASA’s Swift satellite and dubbed GRB 090515. Estimates vary, but short gamma-ray bursts may account for about 10 percent of all bursts. If the magnetar model proves correct, it may indicate that a larger reservoir of stars than previously estimated could power gamma-ray bursts, O’Brien said.

Gamma-ray burst aficionados cite two reasons for their interest in magnetars. First, the enormous rotational energy of these stars, which can spin hundreds of times a second, is sufficient to power some gamma-ray bursts. In addition, a short-lived magnetar could account for the brief but steady X-ray afterglow that immediately follows some long bursts. Unlike a smoldering ember, these unusual X-ray afterglows radiate at a constant brightness until they abruptly die away. The duration of the steady emission matches the lifetime of the magnetar, theorists propose.

In analyzing the afterglow of the short burst GRB 090515, the team led by O’Brien and Rowlinson found that the X-ray emission was unusually steady until it dropped off sharply a few hundred seconds later, just like the afterglows of 10 long gamma-ray bursts they had also recently examined. The team also describes its findings in an upcoming Monthly Notices of the Royal Astronomical Society.

Aside from the magnetar, “there are no other sensible models” to explain a steady X-ray afterglow that suddenly drops to zero, said theorist Péter Mészáros of Pennsylvania State University in University Park. The team’s analysis of GRB 090515 boosts the likelihood that transient magnetars power some short bursts, in addition to some long ones, he said.

Researchers agree that because magnetars have a maximum rate of rotation, these spinning stars are limited to powering bursts less energetic than 3 times 1052 ergs — about the energy that would be unleashed if 3 percent of the sun’s mass were converted into energy. Bursts with higher energies would require a black hole to power them, said Brian Metzger of Princeton University.

More precise estimates of the energy carried by gamma-ray bursts and their afterglows will ultimately determine whether a black hole or a magnetar powers most bursts, said astrophysicist Edo Berger of Harvard University. Those estimates may come as soon as early next year, when Berger begins using an upgraded version of the Very Large Array radio telescope in Socorro, N.M., to study the radio-wavelength afterglow of bursts. Radio afterglows have a particularly simple spherical shape that makes it easy to calculate the energy they contain.

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