Inside the most powerful explosions

New research by an international team that includes Penn State University scientists provides new information about what can happen inside the gigantic bursts of gamma rays that are produced by the catastrophic death of extremely massive stars — the most powerful explosions in the universe. The research has enabled the scientists to begin solving the mystery of whether these gamma ray bursts are the source of extremely high-energy cosmic rays and neutrinos that bombard Earth as astroparticles from space.

The team’s achievement is based on their construction of some of the most sophisticated computational calculations ever that take into account detailed microphysical processes as well as the complex internal structure of gamma ray bursts. The team’s simulations show that emission of the different kinds of astroparticles should be a key to understanding the roles of gamma-ray bursts as extreme particle accelerators. The study also raises new questions that can be answered by next-generation telescopes for the detection of neutrinos and gamma rays. The research will be published online on April 10, 2015, in the journal Nature Communications.

“Gamma ray bursts, the brightest explosive phenomena in the universe, are promising accelerators of very-high-energy particles, with energies much higher than those our current technology can achieve on the Earth,” said Kohta Murase, assistant professor of physics and astronomy and astrophysics at Penn State, a coauthor of the Nature Communications paper along with other scientists from Penn State, Ohio State University, and the DESY national research center in Germany. “Prompt gamma rays are radiated from a relativistic jet, which shoots out into space at velocities that are about 99.9995 percent of the speed of light, leaving behind a newborn black hole or neutron star as a remnant of the massive explosion.”

A gamma ray burst’s jets form when a dying massive star collapses, and powerful plasma streams penetrate their progenitor star through both of its poles. A good fraction of the jets’ energy is converted into energetic particles including gamma rays and neutrinos, which travel far out into space, sometimes for about ten billion light years before reaching Earth. With the new computer calculation built by the research team, the scientists have been able to model details of the production of the very-high-energy astroparticles inside the gamma ray burst’s jets.

The scientists say that this new study is a natural outgrowth of recent findings in astroparticle physics, including the first confirmed cosmic neutrinos detected at the IceCube Neutrino Observatory at the South Pole in 2013. Penn State scientists contributed to this previous discovery.

“Previously, the details of the inhomogeneity of the gamma ray burst jets were not too important in our models, and that was a totally valid assumption — up until IceCube saw the first cosmic neutrinos a couple of years ago,” said Mauricio Bustamante, a Fellow of the Center for Cosmology and AstroParticle Physics at Ohio State and a coauthor of the Nature Communications paper. “Now that we have seen them, we can start excluding some of our initial predictions, and we decided to go one step further and do this kind of analysis.”

The scientists have developed clever techniques to treat the generation and fate of high-energy particles in detail. They wrote new computer code to take into account the shock waves that are likely to occur within the jets. They simulated what would happen when shells of plasma in the jets collided. And they calculated the particle production in each region. In this internal-shock model, some regions of the jet are denser than others, and some plasma shells travel faster than others — like a long highway where the cars are traveling at different speeds. In the gamma ray burst jets, however, the particles are traveling at close to the speed of light.

When these plasma shells collide, they create debris consisting of energetic particles, plus turbulent magnetic fields. “The debris contains neutrinos, cosmic rays, and gamma rays, but, depending on where the collisions occurred, one of these typically will dominate the emission,” Bustamante said. The team’s new calculation shows that, in the internal-shock model, neutrinos largely originate from internal collisions that occur closest to the engine of the gamma ray burst, where the concentration of particles is higher; collisions that occur far away will mostly produce the gamma rays that we detect on Earth; and cosmic-ray protons are mostly released from collisions at intermediate distances from the engine.

The research team’s findings support some ideas developed by Murase, who previously showed the importance of the innermost collisions for the emission of neutrinos. Murase and his collaborators also had suggested that heavier elements like oxygen and iron can be accelerated and emitted as extremely high-energy cosmic rays only if collisions occur sufficiently far away from the engine of the gamma ray burst. The team’s new calculation also implies that the amount of neutrinos that reach the Earth is below the detection threshold that can be achieved by today’s neutrino telescopes such as IceCube.

“We have found a non-trivial new effect that was not shown in any previous work,” Murase said. “Since our predicted fluxes are more robust than previous expectations, our study enhances the feasibility of testing the hypothesis that extremely high-energy cosmic rays come from gamma ray bursts.” When the next generation of neutrino and gamma ray telescopes begin operating, astrophysicists can use this new calculation to refine notions of gamma ray bursts as particle accelerators, and to better understand the sources of extremely high-energy cosmic particles detected on Earth.

In addition to Murase and Bustamante, other co-authors of the paper are Philipp Baerwald at Penn State and Walter Winter at DESY in Germany. This work was funded by NASA, the German Research Foundation, and the U.S. National Science Foundation.”

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