In Powerful Gamma-Ray Bursts, Neutrinos May Fly Out First, Scientists Say

These neutrinos, nearly massless particles that can pass through the Earth unhindered and can penetrate regions of space that choke gamma rays and other forms of light, may carry details of the very first stars to form in the universe. Their presence may also help scientists count the number of massive stars in the universe that have collapsed to form black holes, for many of these collapses may be "dark"--void of signature gamma rays and other telltale radiation, yet flush with neutrinos.

Peter Meszaros of Penn State and Eli Waxman of the Weizmann Institute of Science in Israel publish details of this theory in a recent issue of Physical Review Letters (vol. 87, p. 171102, October 2001).

Gamma-ray bursts are mysterious flashes of gamma rays, the highest-energy form of light. These bursts occur frequently--about once a day, from our vantage point--yet randomly across the sky, lasting for only a few seconds. As such, they are difficult to detect and analyze. Most bursts occur at "cosmological" distances, several billions of light years from Earth from an era when the universe was quite young.

Meszaros said that about two-thirds of all gamma-ray bursts could arise from a fireball formed when the core of a star at least 25 times more massive than the Sun collapses into a black hole. Scientists call such a collapsing star a "collapsar."

In the collapsar model, terrific energy is released as matter pours into a newly formed black hole. A fireball rushes out at near light speed and, due to surrounding stellar pressure, collimates into a jet. This jet smashes into the original star's envelope, which is left behind after the star's core collapsed. If the jet breaks free of the envelope, it produces shock waves that create gamma rays, often by tripping over itself or ramming into other external matter. Scientists recognize this flash of light as the gamma-ray burst.

Yet before the fireball exits the stellar envelope to make gamma rays, Waxman said, it undergoes internal shocks. These shocks accelerate protons, which collide with X-ray photons in the newly forming jet cavity inside the envelope, which in turn create electrons, neutrinos, and anti-neutrinos. The neutrinos punch through the stellar envelope at least ten seconds before the gamma rays are formed.

Furthermore, neutrino bursts can be detected even when there is no gamma-ray burst, Meszaros said. Often, a jet cannot punch through the stellar envelope and create gamma rays--or it might not punch through completely. Regardless, by this point the jet has formed neutrinos, which can easily penetrate the envelope of what Meszaros and Waxman call "choked-off, gamma-ray dark collapses." Thus, neutrino bursts serve as a measure of massive star demise, produced by collapsars that may or may not generate a gamma-ray burst.

This is significant, Waxman said, because the first stars that formed in the universe--beyond redshift 5--might have been far more massive than stars today and, as physics would have it, more likely to be "choked-off, gamma-ray dark collapses," invisible to all detectors other than neutrino detectors.

Meszaros said the AMANDA experiment in Antarctica may soon be able to determine relevant limits on the rate of "dark" as well as "bright" collapses. A cubic-kilometer neutrino telescope called ICECUBE, planned in the Antarctic ice cap as an extension of AMANDA, would provide even greater sensitivity to neutrino bursts.

"Gamma-ray bursts are the strongest known explosions in the universe, but they may be only the tip of the iceberg," Meszaros said. "There could be a far larger number of similarly violent bursts detectable only through their ultra-high-energy neutrinos." These neutrinos would be in the TeV energy range, Meszaros said.