Are gamma-ray bursts standard candles?

Astronomers often liken gamma-ray bursts to cosmic flashbulbs--brief outpourings of high-energy radiation in the sky A new analysis suggests that the mysterious bursts might resemble flashbulbs in more ways than one.

A trio of astrophysicists began by examining 260 gamma-ray bursts detected by NASA's Compton Gamma Ray Observatory (GRO) and charting them on the basis of their intensity If the bursts are distributed uniformly throughout the cosmos, one might expect to find a much larger number of dim flashes than bright ones - just as there exist many more faint, distant stars than bright, nearby ones.

However, cosmology confounds this simplistic model. As the expanding universe moves galaxies farther apart, photons emitted by distant sources lose energy, shifting to redder wavelengths. Some of the photons drop to an energy level below what the GRO detectors can record. The expansion lengthens the amount of time it takes photons from distant bursts to reach Earth, also reducing the number of dim bursts recorded by GRO.

Gordon Emslie of the University of Alabama in Huntsville and his colleagues John M. Horack and Charles A. Meegan of NASA's Marshall Space Flight Center in Huntsville, Ala., were well aware of these cosmological influences when they conducted their graphical analysis. They still expected to find an increase in the number of dim bursts, but at a more gradual rate. Instead, they saw an abrupt slowing of the increase midway between the brightest and dimmest flashes.

Two possible explanations, both of which have startling consequences, can account for his team's results, Emslie notes. A universe that expands at an accelerated rather than a uniform rate could account for the abrupt change in the number of bursts. But an accelerating cosmos would require a repulsive force to counteract gravity This force, which pushes two masses farther apart, would have to grow stronger, not weaker, at larger distances. In mathematical terms, it would require a cosmological constant to modify the accepted theory of gravitation. Other observations don't seem to support this.

Alternatively, he says, gamma-ray bursts may resemble a group of light bulbs, very few of which exceed a certain wattage. If the bursts have a relatively uniform intensity, then the standard model of an expanding universe would account for the new findings, Emslie says. This model would require that the bursts have a narrower range of luminosity than any known group of celestial objects.

The notion that gamma-ray bursts are some type of "standard candle" may provide a new clue for discovering the sources that generate them, he adds. If bursts are standard candles, they might also yield another estimate of the age and size of the universe.