Do supernovas generate gamma-ray bursts?

Astronomers searching for the remnants of the explosions that spawn gamma-ray bursts have a problem. They don't know what they're looking for.

Last week, however, researchers reported progress in their quest to find the remains of bursts, the short-lived but most energetic events in the universe.

Q. Daniel Wang of Northwestern University in Evanston, Ill., made his discovery while examining two expanding shells of gas in the galaxy M101. Astronomers have assumed these to be the remnants of separate stellar explosions called supernovas. During a supernova, a massive star ejects its outer layers and a shock wave sweeps up the material along with surrounding gas and dust. Although powerful, supernovas have been thought to have too little energy to create a gamma-ray burst.

Analyzing the visible-light and X-ray emissions from the two remnants in M101, Wang found that each has 10 to 30 times more energy than a supernova remnant would typically carry. He suggests that both remnants represent material left over from the much more energetic outbursts called hypernovas.

According to theorists, a hypernova arises when a massive star collapses under its own weight to become a black hole. That catastrophic collapse could generate a gamma-ray burst (SN: 5/23/98, p. 326).

Wang says that his observations, presented at a meeting of the American Astronomical Society in Charleston, S.C., represent the first identification of a hypernova remnant.

However, "there is no clear signature that would tell you this is a gamma-ray burst remnant and not something else," comments Abraham Loeb of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass.

In a paper posted on the Internet, Loeb and his Harvard-Smithsonian colleagues suggest ways to search for the remnants of gamma-ray bursts (http://xxx.lanl.gov/ abs/astro-ph/9904181). One clue could be oppositely directed jets of ionized gas--if the bursts are highly focused like a searchlight, as scientists have recently speculated.

Evidence of such jets has led Renyue Cen of Princeton University to argue that supernova 1987A, the nearest supernova to Earth since 1604, may have produced a gamma-ray burst. No one saw the burst because it pointed away from our line of sight, Cen suggests in a paper on the Internet (http://xxx.lanl. gov/abs/astro-ph/9904147).

He bases his proposal on a new analysis of images taken a month after astronomers witnessed the explosion 12 years ago. At that time, Peter Nisenson and Costas Papaliolios now of Harvard-Smithsonian and their colleagues found a mysterious bright spot next to the supernova.

Using a new computer algorithm, Nisenson and Papaliolios confirm the existence of this spot and also find signs of a second spot on the other side of the remnant (http://xxx.lanl.gov/ abs/astro-ph/9904109). The two spots could indicate that 1987A spewed oppositely directed jets of material in addition to generating a bubble-shaped remnant, Cen says.

If these jets prove to be evidence of gamma-ray bursts, then supernovas may be capable of generating the most energetic phenomenon in the cosmos. Because supernovas are far more common residents of the celestial zoo than are hypernovas, this suggests gamma-ray bursts may erupt relatively frequently.

"It's far too early to tell whether this is correct or not," says theorist Bohdan Paczynski of Princeton University, "but it's very tantalizing."