June 9, 1998
Gamma-ray bursts are tremendous releases of energy in the distant cosmos viewed by satellites in earth orbit as brilliant flashes of gamma-ray light, typically occurring at a frequency of one per day. Scientists have long theorized that gamma-ray bursts may result from the in-spiraling and collision of twin burned-out and collapsed stars in close orbit.
Known as binary neutron star systems, the fast orbiting (hundreds of times per second) pairs of stars are estimated to be prevalent enough in the universe to yield the observed one-per-day gamma-ray burst rate. But finding a way to translate the tremendous energy from a binary star collision into an actual gamma-ray burst has proved vexing for theorists.
The report presented today at the American Astronomical Society meeting by Jay D. Salmonson of Livermore Lab and Grant J. Mathews of Notre Dame University hypothesizes that the gamma-ray burst actually occurs moments prior to the collision of the neutron stars.
The basis for the new theory derives from studies by Livermore Laboratory's James Wilson and Mathews. Using computer simulations of neutron binary star systems that incorporate Einstein's general theory of relativity, Mathews and Wilson found a strange effect: The stars are observed to be compressed several seconds before merging. In essence, the general relativistic gravitational effects of two neutron stars rapidly revolving around each other squeezes both stars.
Such squeezing is believed to heat the stars. The dual effects of squeezing and heating causes the stars to radiate a staggering number of elusive sub-atomic particles called neutrinos. A small fraction of the neutrinos will collide, creating innumerable electron-positron pairs.
A super-hot cloud of electron-positron pairs would then expand off of the surfaces of the neutron stars, then rapidly accelerate into space. In the midst of this expanding fireball, the electrons and positrons would in turn collide to create gamma-ray photons. The massive conversion of electrons and positrons to photons would produce the gamma-ray burst.
"We're theorizing that the gamma-ray burst is not the observable effect from the collision of in-spiraling neutron binaries, but the result of heating and the subsequent neutrino-to-photon radiation process that occurs in a few brief seconds before such a collision," said Salmonson, an LLNL/U.C. Davis graduate student.
The gamma-ray burst model explains several basic aspects of the phenomenon: It describes an energy output large enough to match the energy of observed bursts; the duration of the modeled burst agrees with observation data; and the spectrum of the gamma-rays derived from the model is in the range that is characteristic of authentic gamma ray bursts.
The work of Wilson, Mathews and Salmonson is supported by the Department of Energy and the National Science Foundation. Lawrence Livermore National Laboratory is operated by the University of California for DOE.