February 20, 2005

Dying Star Flares Up, Briefly Outshining Rest of Galaxy

or a fraction of a second in December, a dying remnant of an exploded star let out of a burst of light that outshone the Milky Way's other half-trillion stars combined, astronomers announced Friday.

Even on Earth, half a galaxy away, the starburst was one of the brightest objects ever observed in the sky, after the Sun and perhaps a few comets. The magnitude of the event caught most astronomers by surprise.

"Whoppingly bright," said Dr. Bryan M. Gaensler, an astronomer at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass. "It gave off more energy in 0.2 seconds than the Sun does in 100,000 to 200,000 years."

No one on Earth directly saw the flare because most of the light was gamma rays, photons that are more energetic than X-rays and are blocked by the atmosphere. But the Dec. 27 pulse registered on instruments aboard 15 spacecraft, including NASA's new Swift satellite, which was designed to record cosmic gamma rays and had been turned on just the week before.

Dr. Neil Gehrels, the lead scientist for the Swift satellite, said flares of that magnitude could be expected just once in a millennium.

"That seems so improbable it's a puzzle right now," Dr. Gehrels said. "There's something going on here that we don't understand."

The radiation even temporarily compressed Earth's ionosphere, an envelope of charged gas at the top of the atmosphere, and distorted long-wavelength radio signals.

"It was really the big one," said Dr. Kevin Hurley, a researcher at the Space Sciences Laboratory at the University of California, Berkeley. "You could not have missed it."

Dr. Hurley and others pinpointed the origin of the pulse as a neutron star known as SGR 1806-20, about 50,000 light-years distant in the constellation Sagittarius. Neutron stars are remnants of stars after they have exploded in supernovas, and SGR 1806-20 is one of about 10 unusual neutron stars known as magnetars, which have extraordinarily strong magnetic fields, a quadrillion times as strong as Earth's.

Sudden shifts in the intense magnetic fields are believed to generate flares, in much the way the Sun generates solar flares, and two giant magnetar flares had been previously observed, one in 1979 and one in 1998. The Dec. 27 flare, however, was 100 times as powerful.

One physicist, Dr. David Eichler of Ben-Gurion University in Israel, wrote a paper in 2002 suggesting that magnetars might be capable of such cataclysmic flares, but most scientists, Dr. Gaensler said, "had no idea they could make a flare this big."

SGR 1806-20 has almost as much mass as 1.5 Suns, compressed into an incredibly dense ball about a dozen miles wide, and it spins around once every 7.5 seconds, slow for a neutron star. Discovered in 1979, SGR 1806-20 has at times been noisy, firing off small gamma ray flares, and at other times quiet. Its activity picked up in the past year.

"In retrospect, I guess you could say it was getting ready to let go," Dr. Hurley said, adding that he thought the magnetic fields, held in place by the crust of the star, had become twisted, building stress. "At some point, it gives way like an earthquake," he said.

Astronomers presented their observations at a NASA news conference on Friday, and several scientific papers describing the event will be published in a coming issue of the journal Nature.

In the aftermath of the flare, Dr. Gaensler, lead author of one of the Nature papers, and his colleagues used radio telescopes on Earth to track a shock wave radiating from the star. They were surprised to find the wave expanding rapidly, at a quarter of the speed of light, "which is not what you tend to see in the galaxy every day," he said.

SGR 1806-20 itself continues to spin as before, one revolution every 7.5 seconds. "Amazingly, the neutron star is still there," Dr. Gaensler said. "It did not explode or blow itself apart to bits."

The magnetar flare may help solve a cosmic mystery of gamma ray bursts, the prime mission of the Swift satellite. Bursts lasting from several seconds to a couple of minutes are believed to be caused by the collisions of black holes - events that are even more violent than magnetar flares and occur much farther away - but astronomers had been at a loss to explain shorter bursts lasting a couple of seconds or less.

Now they have at least a partial answer: some of the bursts are magnetar flares originating in other galaxies.

"It is clear magnetar flares make short gamma ray bursts," said Dr. Robert C. Duncan of the University of Texas. "It is at least a significant fraction of them."

But that still may not be the whole answer. Dr. Gehrels, the lead scientist for the Swift satellite, said that when astronomers looked in the direction of three recent short gamma ray bursts, those parts of the sky turned out to be empty.

"It just all fits so well, and then there were no galaxies there," Dr. Gehrels said.

But he said that as the satellite observed more gamma ray bursts, "we should know in the next couple of months the answer to this."