Titanic explosions that emit powerful flashes of energetic gamma rays are one of astronomy's hottest mysteries. Now an analysis of the nearest gamma ray burst yet detected has added weight to the popular theory that they are expelled during the death throes of supermassive stars.
Very large stars die spectacular deaths at a relatively young age. Some explode as brilliant supernovas, while the most massive stars probably collapse into black holes. Computer simulations suggest that the birth of a black hole can be accompanied by the release of tremendous amounts of energy, so these "collapsars" are prime candidates as the source of gamma ray bursts. If so, the bursts should occur in star-forming regions, because massive stars die before they are able to escape from their cosmic cradle. Unfortunately, most galaxies where gamma ray bursts originate are billions of light years away, so even the keen-eyed Hubble Space Telescope can't tell if gamma ray bursts occur in star-forming regions.
On 25 April 1998, however, a relatively faint gamma ray burst was detected much closer to home--in a galaxy just 140 million light-years away. Moreover, the burst coincided with a supernova, suggesting a link between gamma ray bursts and the deaths of massive stars. Stephen Holland of the Danish Centre for Astrophysics with the Hubble Space Telescope in Aarhus and his colleagues observed the host galaxy of this gamma ray burst earlier this month with the Hubble.
The Hubble images of the galaxy, known as ESO 184-G82, show that the burst indeed occurred in a giant star-forming region. According to Holland's team, which released the images yesterday, part of the light at the exact burst position may be the fading glow of the supernova, but the surrounding bright objects are either young star clusters or very hot, massive stars.
Titus Galama of the California Institute of Technology in Pasadena, who in 1998 discovered the supernova, says the Hubble observations are "not surprising. This is where you expect [gamma ray bursts] to occur." Team member Jens Hjorth of the University of Copenhagen, Denmark, agrees. "But we've had so many surprises in this field," he says, "that it's good to see that something we expect is actually happening."