Enigmatic Eruptions: Gamma-ray bursts lack supernova fireworks

Ron Cowen

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 http://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.

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Letters:

This article states that gamma-ray bursts are "a million trillion times as bright as the sun." The sun is so bright that humans can't look directly at it from 93 million miles away. How can we possibly wrap our minds around something a million trillion times brighter? Astronomy is great.

Donald Kaufmann
Philadelphia, PA

References:

Cobb, B.E., et al. In press. Could GRB 060614 and its presumed host galaxy be a chance superposition? Astrophysical Journal Letters. Abstract and preprint available at http://arxiv.org/abs/astro-ph/0608678.

Della Valle, M., et al. In press. GRB 060614: An enigmatic long-duration gamma-ray burst not due to a hypernova. Nature. Abstract and preprint available at http://arxiv.org/abs/astro-ph/0608322.

Fynbo, J.P.U., et al. In press. A new type of massive stellar death: No supernovae from two nearby long gamma ray bursts. Nature. Abstract and preprint available at http://arxiv.org/abs/astro-ph/0608313.

Gal-Yam, A. . . . S.R. Kulkarni, et al. In press. The g-ray burst GRB060614 requires a novel explosive process. Astrophysical Journal Letters. Abstract and preprint available at http://arxiv.org/abs/astro-ph/0608257.

Schaefer, B.E., and L. Xiao. In press. GRB060614 is at high redshift, so no new class of gamma-ray bursts is required. Astrophysical Journal Letters. Abstract and preprint available at http://arxiv.org/abs/astro-ph/0608441.

Sources:

Massimo Della Valle
INAF
Observatorio Astrofisico di Arcetri
Largo E. Fermi 5
I-50125 Firenze
Italy

Johan P.U. Fynbo
Dark Cosmology Centre
Niels Bohr Institute
University of Copenhagen
Juliane Maries Vej 30
DK-2100 Copenhagen
Denmark

Shrinivas Kulkarni
Division of Physics, Mathematics and Astronomy
California Institute of Technology
Pasadena, CA 91125

Andrew MacFayden
Institute for Advanced Study
School of Natural Sciences
Princeton, NJ 08540

Bradley E. Schaefer
Department of Physics and Astronomy
Louisiana State University
Baton Rouge, LA 70803

Stan Woosley
University of California, Santa Cruz
Department of Astronomy & Astrophysics
Santa Cruz, CA 95064

From Science News, Volume 170, No. 13, September 23, 2006, p. 196.