Research links magnetism, gamma-ray burst phenomenon

Rice-Los Alamos team find remarkable similarity between data from sky and computer

In the early years of the Space Age, astronomers made the startling discovery that short, transient flashes of gamma rays occurred randomly in the sky every night. Only within the past decade have scientists uncovered evidence to associate gamma-ray bursts with the death cries of massive stars from the edge of the universe. But they’ve had very few clues about how a “hypernova” or “collapsar” might produce such energetic bursts.

New findings from Rice University and Los Alamos National Laboratory (LANL) about a previously undiscovered particle acceleration mechanism indicate that strong magnetic fields may play a crucial role in the formation of gamma ray bursts. The research is published in today’s issue of the journal Physical Review Letters.

“When we compared the signatures in energy and in space-and-time of the particles accelerated by the newly discovered mechanism, called the ’diamagnetic relativistic pulse accelerator’, with the telltale signatures of cosmic gamma-ray bursts, we found that they were remarkably similar,” said paper co-author Edison Liang, the Andrew Hays Buchanan Professor of Astrophysics at Rice.

The diamagnetic relativistic pulse accelerator is a process that occurs when a bubble of strong magnetic field and electron-positron plasmas. (Positrons are the positively-charged “antimatter” version of electrons) is suddenly released and allowed to expand outward near the speed of light. The resulting explosion creates an intense electromagnetic pulse together with an intense electrical current. This electrical current, also called a “drift current” because it flows perpendicular to the magnetic field, then helps to trap the expanding plasma near the surface and accelerate it. Most of the magnetic energy is eventually converted into fast-moving particles travelling near the speed of light. These particles then radiate away their new-found energy as gamma rays.

Liang and co-author Kazumi Nishimura of LANL began collaborating on computer simulations of electromagnetic explosions two years ago, but their latest simulations were the first that looked at the long-term behavior of the plasma pulse created by the explosion. They discovered three unusual physical properties. First, rather than creating a large, single burst of energy, the plasma pulse divided repeatedly over time, creating a rapid succession of smaller energy bursts — successions that bear striking similarity to the patterns of gamma-ray bursts seen in space. Second, the energies of the particles blown out by the electromagnetic explosion are distributed in a unique way that is also similar to the gamma-ray energy distributions found in cosmic gamma-ray bursts. The final and most dramatic finding is that there is a simple mathematical relation between the average energy of the accelerated particles, the strength of the magnetic field and the time they have been expanding since the explosion. When this equation is applied to gamma-ray bursts, it gives results that are also consistent with observed data.

“It’s widely agreed that most gamma-ray bursts result from gigantic explosions related to stellar deaths, but the scientific community has been divided over the precise way the bursts are formed,” said Liang. “Some believe they’re created through non-magnetic hydrodynamic explosions, while others believe that magnetic field plays a key role. Until now, neither side could offer a concrete mechanism that naturally explains these unique properties. We believe we’ve demonstrated that in these computer simulations.”

Liang and Nishimura’s computer simulation only describes how an exploding bubble of strongly magnetized electron-positron plasma might lead to bursts of gamma rays similar to those seen in the sky. The simulation does not address how such a magnetic bubble might form in the heart of a dying star, but others groups are working on such models, said Liang.

“For example, matter spiraling into a newly formed black hole located at the heart of a dying star could wind up the magnetic field of the stellar core to form such bubbles,” said Liang. “Like air bubbles in the ocean, these bubbles of magnetized plasma, also called “Poynting flux”, would have a tendency to rise to the surface of the star at near the speed of light. Suddenly freed from the confining pressure of the star’s envelope, these bubbles would expand and launch the pulse accelerator process.”

NASA and the Department of Energy funded the research.