Scientists have created a simulation that proves how some of the most powerful explosions in the universe, hypernovae, can be responsible for some of its brightest and most mysterious events, gamma ray bursts (GRBs). Incredibly, this is all based on just the first 10 milliseconds (10 thousandths of a second) after a massive star collapses.
The research was led by scientists at the University of California, Berkeley, and is published in Nature. The process itself involves a rapidly rotating star collapsing. As this happens, it spins faster and faster with its attached magnetic field, producing a “dynamo” effect that is a million billion times stronger than Earth’s magnetic field.
“A dynamo is a way of taking the small-scale magnetic structures inside a massive star and converting them into larger and larger magnetic structures needed to produce hypernovae and long gamma-ray bursts,” said Philipp Mösta, a UC Berkeley postdoctoral fellow and first author of the paper, in a statement. “People had believed this process could work out. Now we actually show it.”
Hypernovae are extremely powerful supernovae – stellar explosions – but their cause is not fully understood. In a hypernova, the inner star that is about 930 miles (1,500 kilometers) across collapses into a neutron star about 10 miles (15 kilometers) across, known as a core collapse.
GRBs, meanwhile, are among the brightest events in the universe, hugely powerful emissions of gamma rays of unknown origin lasting up to 100 seconds, while hypernovae shine more than 10 times brighter than an average supernova.
Crucially, the simulation helps to explain a “missing link” in connecting hypernovae with GRBs. Scientists had been unsure how a star could amplify a magnetic field not wholly dissimilar to the Sun’s in terms of power into one a quadrillion times more powerful during these explosive events.
This supercomputer visualization shows how a star’s rotation can rev up its magnetic field to a million billion times the power of our Sun’s. UC Berkeley Campus Life
The key appears to be a “shear zone” 10 to 20 miles (15 to 35 kilometers) from the inner star where its different layers are rotating at different speeds, creating a large amount of turbulence that causes the dynamo effect and leads to the hugely amplified magnetic fields. These in turn can support two jets in opposite directions composed of extremely energetic gamma rays, namely gamma ray bursts.
In this simulation, 130,000 computer cores at the Blue Waters supercomputer at the National Center for Supercomputing Applications at the University of Illinois at Urbana-Champaign were used to model the brief fraction of a second after the core collapse, producing the intriguing results.
“The breakthrough here is that Philipp’s team starts from a relatively weak magnetic field and shows it building up to be a very strong and large-scale coherent magnetic field of the kind that is usually assumed to be there when people make models of gamma-ray bursts,” said Eliot Quataert, a UC Berkeley professor of astronomy who was not involved with the study, in the statement.
The simulation shows how the dynamo effect causes a feedback loop that can create huge magnetic fields when a massive star collapses, producing both cosmic phenomena. Future simulations from the same team will seek to model more than just 10 milliseconds of a hypernova’s evolution to further understand the process taking place.
