Some gamma-ray bursts may occur nearby

Gamma-ray bursts, the flashes of high-energy light produced by the most powerful explosions in the cosmos, originate in galaxies billions of light-years from Earth. That’s been the assumption since the late 1990s, when astronomers began measuring the distances to a dozen or so of these fleeting events.

Map of 70 gamma-ray bursts shows that they concentrate along the supercluster of galaxies containing the Milky Way. Green indicates Virgo, the cluster that houses our galaxy. Norris et al.

But a provocative new study hints that a larger-than-expected number of these titanic explosions come from galaxies that lie within a few hundred million light-years of Earth. If enough gamma-ray bursts do indeed emanate from relatively nearby galaxies, researchers may be able to uncover exactly how these mysterious flashes arise, notes study collaborator Jay P. Norris of NASA’s Goddard Space Flight Center in Greenbelt, Md.

A nearby group of gamma-ray bursts would have another attraction. Theorists have proposed that the bursts are produced when massive stars merge and collapse into black holes (SN: 7/10/99, p. 28) in a process that would yield gravitational waves. When sensitive detectors come on-line in about 5 years, the first waves they detect could be those from nearby bursts.

The findings by Norris’ team depend on a novel, as yet unconfirmed, method of estimating the distance to gamma-ray bursts. The method examines links between several properties of the bursts. For starters, researchers have long recognized that these events emit most of their highest-energy photons before they emit lower-energy photons. This lag ranges from microseconds to seconds.

Using that time-lag information, Norris’ team examined nine bursts for which astronomers have measured the spectra of their home galaxies and thereby their distances. The researchers found that the bursts with the longest lags were intrinsically the dimmest, as if they were extremely low-wattage light bulbs.

If the relation between photon lag and brightness holds true in general, it would enable scientists to estimate the distances to the thousands of bursts that have not been directly measured. By comparing the intrinsic brightness of a burst–based on the time lag between the high- and low-energy photons–with how bright the burst appears in the sky, researchers could determine how far away it is.

Norris’ team homed in on the 70 gamma-ray bursts with the longest time lags–2 seconds or more–recorded by NASA’s Compton Gamma Ray Observatory. According to the proposed relation, the 70 bursts should be intrinsically dim. That suggests they must have exploded relatively nearby, since the observatory could not have discerned dim bursts originating far away.

Other evidence supports that conclusion, Norris reported last week at a meeting of the American Astronomical Society in Washington, D.C. A map of the gamma-ray bursts shows that they congregate along the plane of a galactic supercluster that includes the Milky Way, indicating that the bursts lie within 325 million light-years of Earth.

Those findings are “going to be debated at gamma-rayburst meetings now for awhile,” notes Robert J. Nemiroff of Michigan Technical University in Houghton. The Swift gamma-ray observatory, to be launched in 2003, may settle the issue. It can detect extremely dim bursts and determine their distance directly.

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