Evidence of supernova coming to light

4/10/2003

Call it Act II of “A Star is Dying.''

Act I took place two weeks ago, when the brightest gamma-ray burst ever to blast the Earth caught the attention of astronomers around the globe. Everyone waited to see what would happen next. The second act might reveal the culprit behind this super-energetic blast of invisible light, a subject that has taunted astronomy for decades.

Now, the evidence is in. A group of researchers using a 6.5-meter telescope (about 21 feet) in Arizona announced capture of the guilty party's definitive fingerprint: The blast of gamma rays March 29 was the first thrashings of a star exploding in a supernova 2 billion years ago.

Just days after the gamma-ray blast, a team from the Harvard-Smithsonian Center for Astrophysics and the University of Arizona pointed the telescope at the source of the blast in the constellation Leo. If this thing was a supernova, they wanted to catch the first evidence.

“We started seeing something Sunday,'' said Krzysztof Stanek of Harvard. “Then Tuesday, we analyzed our data from Monday night. There is no doubt. It's a supernova.''

Astrophysicists predicted that the glow of a supernova would follow what Dr. Stanek is calling “The mother of all gamma-ray blasts.'' The predominant theory suggests that such blasts come from a rapidly spinning giant star as it collapses into itself. As the star spins, it shoots a column of energy from its poles in the form of gamma rays. In 2001, Dr. Stanek and others saw evidence of a supernova in the afterglow of a gamma-ray burst, but they were unable to conduct spectroscopy - a recording of the exact light wavelengths - on the event to nail it.

“It was good evidence, but missing the spectroscopy lines, people could argue. It's hard to argue against this one,'' Dr. Stanek said.

“When you only have one, it could always be a coincidence that a supernova was in the same line of sight as a gamma-ray burst,'' said Jay Norris, who researches gamma rays at the NASA Goddard Space Flight Center in Greenbelt, Md.

“The question for me now is, will we see wonderful evidence of the underlying supernova, and I think that's here,'' Dr. Norris said. “I think that's what I'm seeing. I wouldn't bet my life on it, but if I had to bet one way or the other, I'd bet on it.”

Astronomers didn't see the supernova's signature immediately because it's overwhelmed by the brightness of the gamma-ray's energetic afterglow. As the afterglow dims, and the supernova signature grows, its chance of detection increases. Plus, supernova explosions take as long as two weeks to reach their full extent. The supernova researchers watch today still may be visible a month from now.

Although the supernova was preceded by a record-breaking gamma-ray blast, the star explosion itself is too dim to see without a very powerful telescope. The brightness of the gamma rays from this dying planet was most likely due to the direction the collapsing star was pointing. As it shot gamma rays from its poles, it was pointing right at us.

“We're only seeing one in a few hundred of the gamma-ray bursts that occur in the universe every day,'' said Dr. Norris. “We're only seeing a small fraction because the rest are not jetted to us.''

Craig Wheeler, the University of Texas astronomer who's watching for supernova signs with the McDonald Observatory's nine-meter (30-feet) Hobby-Eberle Telescope, fought technical troubles, and then weather, forcing his team to miss the first signs of the supernova eruption. But he agrees the data from the Arizona telescope “looks in detail like a supernova.''

Still, it leaves plenty of mysteries to solve. It says nothing about the sources of all those other gamma-ray bursts in the universe. In fact, a gamma-ray burst he studied in October was nothing like this one. Instead of preceding a supernova, it followed one.

“That's a model I fought kicking and screaming,'' he said.

Still other gamma-ray bursts may come from colliding neutron stars - neutron stars are very dense stars formed when some stars collapse. And other blasts may come from the birth of a black hole unaccompanied by a supernova, Dr. Wheeler said.

“Every one is different, every one is exciting,'' he said.

As one astronomer in the field is often quoted: “If you've seen one gamma-ray burst, you've seen one gamma-ray burst.''