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 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.

To receive a copy of the paper about the neutrino-burst theory published
in Physical Review Letters, refer to <http://prl.aps.org/>
and get the article Phys. Rev. Lett., vol. 87, page #171102, or contact
<science@psu.edu>. For information
about the AMANDA detector, refer to <http://amanda.berkeley.edu>.
For background information on gamma-ray bursts, refer to <http://swift.sonoma.edu>.

< Christopher Wanjek, GSFC, for Penn State >

Contact:

Peter Meszaros at Penn State: phone +814-865-0418, e-mail <pmeszaros@astro.psu.edu>
Eli Waxman at the Weizmann Institute: phone +972-8-934 426, e-mail
<waxman@wicc.weizmann.ac.il>
Barbara Kennedy at Penn State (PIO): phone +814-863-4682, e-mail
<science@psu.edu>
Yivsam Azgad (PIO) at the Weizmann Institute (Israel): phone +972-8-934
3856 , e-mail <yivsam@wisemail.weizmann.ac.il>
Jeffrey Sussman (PIO) at the Weizmann Institute (New York): phone
+212-895-7951, e-mail <jeffrey@acwis.org>