National Astronomical Observatory of Japan

Simulations Find Mechanism of Brightest Flashes in Universe

| Science

Artist’s impression of a high-speed jet. The close-up shows how the expansion of the gamma-ray burst jet enables gamma rays (represented by white dots) to escape. Original size (2.2MB)

Scientists have used simulations to show that the photons emitted by long gamma-ray bursts, the most powerful electromagnetic phenomena in the Universe, originate at the visible surface of high-speed jets emitted by exploding stars.

Gamma-ray bursts release as much energy in a second or so as the Sun will release over its entire lifetime. Scientists now know that one of the types, long bursts, originate from high-speed jets of matter ejected during the deaths of massive stars. However, exactly how the jets produce gamma rays was still a mystery.

A team led by Hirotaka Ito of the RIKEN Cluster for Pioneering Research investigated this mystery paying special attention to the Yonetoku relation, a tight correlation between the spectral peak energy and peak luminosity of long gamma-ray bursts. Using computer simulations performed on several supercomputers, including ATERUI and ATERUI II of the National Astronomical Observatory of Japan, the group focused on the so-called “photospheric emission” model. In this theory the photons we observe are emitted from the photosphere, the visible surface of the jet. As the jet expands, it becomes easier for photons to escape. Thus the photosphere where it becomes possible for photons, including gamma rays to escape, moves downward through the jet to material that was originally denser.

Using three-dimensional simulations and radiation transfer calculations, they were able to determine that the model worked for long gamma-ray bursts. Their simulations revealed that the Yonetoku relation is a natural consequence of the jet-star interactions. “To us,” says team leader Ito, “this strongly suggests that photospheric emission is the emission mechanism of gamma-ray bursts. ... there are still mysteries concerning how the relativistic jets themselves are generated by the collapsing stars. Our calculations should provide valuable insights for looking into the fundamental mechanism behind the generation of these tremendously powerful events.”

These results appeared as Ito et al.“ The photospheric origin of the Yonetoku relation in gamma-ray bursts,” in Nature Communications on April 3, 2019.

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