NRL Scientists Report New Findings About Cosmic Rays

1/6/2004 - 1-04r
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Cosmic Rays Made by Gamma-Ray Burst Explosions That Create Black Holes

Scientists at the U.S. Naval Research Laboratory say that gamma-ray bursts, the most powerful explosions known, are the engines behind the creation of high-energy cosmic rays, providing a complete solution to an enduring mystery in astronomy.

The unusual spectrum of light from a gamma-ray burst detected nearly ten years ago (but only recently analyzed) gives strong evidence that such explosions, which likely also produce a black hole, directly accelerate these enigmatic cosmic rays.

Drs. Charles Dermer and Stuart Wick of NRL present these finding in separate talks today and Thursday at the American Astronomical Society meeting in Atlanta, Georgia. Their colleague on this work, Dr. Armen Atoyan, is in the Mathematical Research Center (Centre de Recherches Mathématiques) at the University of Montreal.

Cosmic rays, discovered over 90 years ago, are nuclear and elementary particles such as protons, nuclei and electrons that hurl through space. The lowest energy cosmic rays, called Solar Energetic Particles, are largely electrons and ions that are accelerated and flung from the Sun during solar flares. Higher energy cosmic rays, traveling a whisker shy of light speed, come from beyond the Solar System. Many of these cosmic rays are likely to have been accelerated by stellar explosions called supernovae.

The origin of the highest-energy cosmic rays, which have energies thousands to millions of times more powerful than can be made in laboratory particle accelerators, has defied explanation in terms of supernova explosions. Unfortunately, tracing the direction of a cosmic ray back to its source is futile because, unlike light, the paths of cosmic rays are twisted by magnetic fields throughout the universe.

Scientists in the past have proposed that the highest-energy cosmic rays originate from rapidly spinning neutron stars in the Milky Way, from the activity of super-massive black holes in other galaxies, and even from exotic physical systems involving dark matter and extra dimensions.

"Most of the accepted theories for cosmic rays require supernovae and shocks," said Dermer. "But they run into problems at high energies. An explosion that creates a black hole and powers a gamma-ray burst can overcome these problems."

Cosmic rays come in a variety of energies, which is measured in electron volts, eV. For comparison, the visible light particles (photons) that the Hubble Space Telescope detects are in the 1 to 100 eV range. X rays are typically thousands of electron volts, in the keV range. Solar cosmic rays are roughly between a million eV (MeV) to a billion eV (GeV). Medium-energy cosmic rays have energies up to hundreds of trillions of electron volts, and high-energy cosmic rays have energies above 1014 eV. Ultra-high energy cosmic rays are greater than 1018 eV, and the highest-energy cosmic ray ever detected had over 1020 eV of energy. A single cosmic-ray particle at this energy carries about the same kinetic energy as a major league fastball.

In short, the NRL team reports at the AAS meeting and in an article accepted for publication in the journal Astroparticle Physics, that high-energy cosmic rays arise from gamma-ray bursts within our Milky Way galaxy and that ultra-high-energy cosmic rays, up to a thousand times more powerful, are particles that come from beyond our Galaxy.

In the model proposed by Wick, Dermer and Atoyan, stars that collapse to form neutron stars power the medium-energy cosmic rays, and stars that collapse to form black holes form the high-energy cosmic rays.

Key to this result was the creation of a model that describes how a gamma-ray burst can accelerate particles to such high energies in the explosions that form black holes. This model can explain observations by Dr. Brenda Dingus of Los Alamos National Laboratory and her University of Wisconsin graduate student, Ms. Magda González, of a gamma-ray burst that occurred October 17, 1994, and thus named GRB 941017.

The radiation spectrum from this event, measured with instruments onboard the Compton Gamma Ray Observatory, had an unusually long-lived and energetic spray of gamma rays seen well after the main portion of the burst. Dermer and Atoyan argue that this delayed radiation can be explained by the presence of a collimated neutron beam formed from cosmic rays accelerated by the gamma-ray bursts. These neutrons, moving near light speed, would evolve into a beam of hyper-relativistic electrons that, in turn, would radiate gamma rays at the energies observed.

The presence of neutrons implies the presence of protons as well, which are in essence the high-energy cosmic rays. Research led by Wick builds upon the connection between cosmic rays produced locally in the Milky Way galaxy and those produced far away in the distant galaxies that have lost energy in their journey through the universe. (These cosmic rays can collide and lose energy with light particles from the afterglow of the Big Bang, called the cosmic microwave background.) Wick applied statistical modeling to differentiate between galactic and extragalactic cosmic rays.

The scenario proposed by Wick, Dermer and Atoyan fits cosmic-ray data from 100 TeV through the so-called knee of the cosmic ray spectrum to the highest energies, with a unified origin of all cosmic rays from supernovae and gamma-ray bursts. Crucial to testing this model will be observations by the NASA Swift gamma-ray burst satellite, scheduled for launch in mid-2004, and the NASA/DoE Gamma ray Large Area Space Telescope (GLAST), scheduled for launch in 2007.

This model also predicts that high-energy neutrinos will be detected from gamma-ray bursts like GRB 941017 with the National Science Foundation's IceCube neutrino experiment, now being built at the South Pole.

This work is supported by the Office of Naval Research and NASA.

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