Gamma-ray bursts: farther and brighter?

Typically lasting just 10 seconds, a gamma-ray burst unleashes more energy than the sun will in its 10-billion-year lifetime. A new study suggests that these mysterious flashes are even more dazzling than many astronomers had imagined -- they may be 20 times more energetic, and some may originate from the first galaxies in the cosmos.

That's the consensus reached by a group of astrophysicists after analyzing the properties of hundreds of bursts.

The team adopted the widely held belief that these ephemeral flashes are linked to the demise of massive stars. Heavyweight stars live life in the fast lane, burning brightly but dying quickly. Because they die so soon after they are born, the death rate of these stars -- and by association, the generation of gamma-ray bursts -- should closely track the rate of star formation in the universe, the researchers reasoned.

Over the past few years, detailed surveys from several telescopes have enabled astronomers to map the history of star formation. These surveys suggest that the universe produced more than half its stars by the time it was half its current age.

Matching the inferred rate of star formation to both the rate and spectra of observed gamma-ray bursts, and assuming that gamma-ray bursts all have about the same intrinsic luminosity, Ralph A.M.J. Wijers of the University of Cambridge in England and his colleagues calculate that radiation from the bursts that appear dimmest has traveled for 15 billion years -- nearly the entire age of the universe. If true, these flashes would beat out quasar light as signs of the most distant known objects.

"If our result is correct, we might be able to detect gamma-ray bursts from the deaths of the first stars that ever formed and perhaps find out when they formed," says study collaborator Jasjeet S. Bagla of Cambridge. Wijers and Bagla, along with Joshua S. Bloom of the California Institute of Technology in Pasadena and Priyamvada Natarajan of the University of Toronto's Canadian Institute of Theoretical Astrophysics, describe their work in the Feb. 11 Monthly Notices of the Royal Astronomical Society.

Although spacecraft have recorded gamma-ray bursts since the 1960s, researchers had no proof until last year that any of the bursts were associated with galaxies even a few billion light-years distant (SN: 3/22/97, p. 174). If many of the bursts come from galaxies farther away, their faintness could explain why only a few host galaxies have been found.

The researchers "have pieced together a scenario that accords with the data, but it might not be the whole story, and it might not even be the right story," notes gamma-ray theorist Charles D. Dermer of the Naval Research Laboratory in Washington, D.C.

Dermer estimates that in at least 1 of every 100 galaxies, a gamma-ray burst should have occurred within the past million years. If Wijers and his colleagues are correct, the interstellar medium in these galaxies should bear obvious scars from the energitic eruptions, which are equivalent to the simultaneous explosion of 1,000 supernovas, he notes.

In one popular model, the dense remnants of two dead stars collide to create a gamma-ray burst. The stars may take so long to spiral toward each other that the burst rate becomes divorced from the star formation rate, Dermer believes.

Nonetheless, he says, the new study merits follow-up and may spur the development of sensitive detectors to study the weakest, presumably most distant flashes.