Spring 2002 -- Vol. 19, No. 1

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

The most powerful explosions in the universe, gamma-ray bursts, may come with a 10-second warning: an equally violent burst of ultra-high-energy particles called neutrinos.

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 Mészáros of Penn State and Eli Waxman of the Weizmann Institute of Science in Israel published details of this theory in a the October issue of Physical Review Letters.

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.

Mészáros 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 interna 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, Mészáros 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 Mészáros 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. Mészáros 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.

~~ Christopher Wanjek, NASA

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