Explosive News: Telescopes find signs of gentler gamma-ray bursts

Some of the most powerful explosions in the universe could be 10 times as abundant as astronomers had assumed. That suggestion comes from two new studies indicating that many gamma-ray bursts—intense flashes of gamma-ray photons—go undetected because they don’t pack quite as much punch as do the bursts that astronomers have recorded for years.

POWERHOUSE. In this model of a gamma-ray burst, jets of high-energy particles emerge from a rapidly rotating black hole (center) and then slam into surrounding matter. M. Weiss/CXC

According to a leading theory, gamma-ray bursts arise when a dying star collapses to become a rotating black hole or a neutron star. The gamma rays emerge when particles jetting from a doughnut-shaped disk surrounding the collapsed star plow into surrounding space.

In the most basic version of that scenario, all gamma-ray bursts have about the same amount of energy. If so, the energy detected at Earth would depend only on a burst’s distance, the width of the jet, and how much of it is aimed toward our planet.

However, “the idea that all gamma-ray bursts spit out the same amount of gamma rays . . . is simply ruled out by the new data,” says Sergey Yu Sazonov of the Space Research Institute of the Russian Academy of Sciences in Moscow and the Max Planck Institute for Astrophysics in Garching, Germany.

Sazonov and his collaborators, as well as a team led by Alicia Soderberg of the California Institute of Technology in Pasadena, report their findings in the Aug. 5 Nature.

Both teams began their work last Dec. 3, when the European Space Agency’s satellite INTEGRAL (International Gamma-Ray Astrophysics Laboratory) recorded an unusual gamma-ray burst, dubbed GRB 031203 for the date on which it was detected.

Just 18 seconds after INTEGRAL recorded the burst, the satellite electronically notified a network of observatories, so astronomers could pinpoint the burst’s location in time to record a rapidly fading X-ray afterglow. Soderberg and her colleagues went further, recording the afterglow with radio and visible-light telescopes.

The data revealed that the gamma-ray burst had emanated from a relatively nearby galaxy—about 1.3 billion light-years away. Yet despite the burst’s proximity, which presumably would enable telescopes to capture most of its gamma rays, the burst was only about one-thousandth as energetic as those originating in much more distant galaxies.

The researchers say that they can’t account for the lower energy by assuming that most of the burst pointed away from Earth. For example, the afterglow of a burst spreads out over time, so the observed afterglow of a burst pointing away should rise rapidly, but the afterglow of GRB 031203 showed no such increase. “It therefore appears that this was much more likely a truly low-energy event,” Sazonov says.

Astronomers already had tentative evidence that another gamma-ray burst, recorded in 1998, exploded from a nearby galaxy and was less energetic than the others that had been observed (SN: 7/10/99, p. 28). Combining the 1998 data with the new, more definitive finding, “we are now thinking that there may be many more subenergetic bursts,” says Soderberg. Too faint to be detected unless they lie close to Earth, low-energy bursts could amount to as much as 10 times the number of bursts bright enough to be routinely recorded by current telescopes, she estimates.

A NASA satellite called Swift may unveil many of these hidden explosions, says Stan Woosley of the University of California, Santa Cruz. Scheduled for launch in October, Swift will detect fainter bursts and localize them more quickly than other gamma-ray observatories do.

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