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Science Snapshot: Simulations Explore Physics of Gamma Ray Bursts

Artist's conception of a gamma-ray burst. The GRB is visible from Earth if the jets (yellow) are oriented so that one points toward us. (Image courtesy of NASA.)

Gamma-ray bursts are the most energetic events in the universe. They involve physics under conditions that can't be achieved anywhere else. As their name implies, their defining characteristic is a temporary flash of gamma rays, the most energetic photons of light.

GRBs are likely to originate during the formation of black holes, when intense beams of particles are shot out following the collapse of a giant star or the merger of two extremely massive stars called neutron stars. Thus GRBs begin as extreme particle accelerators! The details of what actually happens to go from highly accelerated particles to the visible GRB can be studied in detail only through computer simulations, since these extreme energies and huge sizes are beyond the reach of any laboratory experiment.

Jonathan McKinney, a postdoc at the Kavli Institute for Particle Astrophysics and Cosmology at SLAC and Stanford, uses such simulations to explore how the so-called "prompt" emission in GRBs comes to be. Typically a GRB has a "prompt" emission lasting a few seconds, where researchers see the flash of gamma rays, followed by a much longer "afterglow" of visible, radio, and X-ray light. The simulations start from the laws governing the physics of very high-energy charged particles and very strong magnetic fields, of the type predicted to be present in GRBs. They then follow subatomic particles, fields and radiation as they all interact according to those rules.

There are two proposed candidates for the mechanism behind prompt emission: "internal shocks," which happen when bunches of the ultra-fast-moving particles run into each other, and "electromagnetic dissipation," where vast quantities of energy are released when intense electromagnetic fields twist up and spring free—a much higher-energy analog of solar flares. In a recent paper, McKinney and colleague Dimirty Uzdensky of the University of Colorado describe detailed simulations exploring how dissipation could give rise to observed characteristics of the prompt emission, such as its duration, intensity and the spectrum of light it contains. They show that electromagnetic dissipation could be an important factor in prompt emission.

Such simulations are a powerful and necessary tool for exploring not just the astrophysics of GRBs, but other high-energy physics environments, such as the plasmas, where fusion can occur. Developing the capabilities to understand the physics in GRBs is an important piece of a larger effort to predict the behavior of plasmas in general.

This work is based on a paper submitted to Monthly Notices of the Royal Astronomical Society and available at arXiv:1011.1904.

—based on a KIPAC research highlight by Jack Singal
SLAC Today, March 10, 2011