Enigmatic eruptions: gamma-ray bursts lack supernova fireworks.

Gamma-ray bursts, the most powerful explosions in the universe, just got more mysterious. New observations challenge the theory that astronomers had constructed for the origin of many of these cosmic flashbulbs.

These flashes of high-energy radiation are a million trillion times as bright as the sun. According to the leading theory, any gamma-ray burst lasting more than 2 seconds is associated with a supernova, the explosive death of a massive star. A high-speed jet of material emerging from the collapsing star, which becomes either a neutron star or a black hole, generates the burst. At about the same time, a wind or shock wave carrying radioactive nickel-56 powers the visible supernova.

Most bursts are too far away for telescopes to see the underlying supernovas, but stellar explosions have been identified for four of the closest known bursts.

Now, however, three teams report online (see www.sciencenews.org/articles/ 20060923/fob3ref.asp) that two bursts, seemingly nearby, appear to have no supernova associated with them.

One team studied a burst recorded on May 5, while all three groups examined a burst seen on June 14. In each case, the researchers assert that the burst originated from a galaxy relatively close to the Milky Way.

In a thorough search with several telescopes, the groups failed to find a supernova associated with the June 14 event. "This is the first clear evidence that long-duration gamma-ray bursts can be either associated with supernovas which do not have [the usual] properties or they can be produced by a different phenomenon," note Massimo Della Valle of the INAF-Astrophysical Observatory of Arcetri in Firenze, Italy, and his colleagues.

Another group, which includes Shri Kulkarni of the California Institute of Technology in Pasadena, suggests that the June 14 burst may belong to a previously unknown class. A third team, which includes Johan Fynbo of the University of Copenhagen, concludes that both bursts have enigmatic origins.

Two theorists who developed the prevailing models for long-duration gamma-ray bursts say that there may be a simple explanation. Associated supernovas perhaps weren't visible because not enough nickel-56 was available during the explosions, says Andrew MacFadyen of the Institute for Advanced Study in Princeton, N.J. Either too little of the isotope is produced or it falls into the newly formed black hole, he says.

Stan Woosley of the University of California, Santa Cruz agrees. "The evidence is quite solid that most of the long gamma-ray bursts seen so far come from the deaths of massive stars," he says.

The findings highlight the diversity of stellar explosions, MacFadyen adds.

Meanwhile, two papers posted online by a team at Yale University and by Brad Schaefer of Louisiana State University in Baton Rouge claim that the June 14 gamma-ray burst is much farther away than the three teams reported. But Kulkarni and his collaborators calculate they are 98.5 percent certain that the nearby galaxy is the burst's home.