Simulations Find Mechanism of Brightest Flashes in Universe

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.

Figure 1: 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. (Credit NAOJ)
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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.

Figure 2: 3D profile with a 2D slice taken through the midplane of the simulation. The jet axis (dashed arrow) and our viewing angle from Earth (dotted line) are also shown. (Adapted from Ito et al. (2019) Nature Communications)
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Figure 3: Graph comparing observations and simulation results to the Yonetoku relation. The different simulations use different jet powers and times. (Adapted from Ito et al. (2019) Nature Communications)
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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.

[Paper Details]

Title: The photospheric origin of the Yonetoku relation in gamma-ray bursts
Journal: Nature Communications
Authors: Hirotaka Ito1,2, Jin Matsumoto3, Hiroshige Nagataki1,2, Donald C. Warren2, Maxim V. Barkov4, Daisuke Yonetoku5
1) Astrophysical Big Bang Laboratory, RIKEN, 2) Interdisciplinary Theoretical & Mathematical Science Program (iTHEMS), RIKEN,3) School of Mathematics, Faculty of Mathematics and Physical Sciences, University of Leeds, 4) Department of Physics and Astronomy, Purdue University, 5) College of Science and Engineering, School of Mathematics and Physics, Kanazawa University

This work was supported by JSPS KAKENHI Grant Number JP16K21630 and JP16KK0109. Numerical computations and data analysis were carried out on ATERUI and ATERUI II at Center for Computational Astrophysics, National Astronomical Observatory of Japan, the Yukawa Institute Computer Facility and Hokusai BigWaterfall system at RIKEN. This work was supported in part by a RIKEN Interdisciplinary Theoretical & Mathematical Science Program (iTHEMS) and a RIKEN pioneering project "Extreme precisions to Explore fundamental physics with Exotic particles (E3-Project)". This work was supported by NSF grant AST-1306672, DoE grant DE-SC0016369, and NASA grant 80NSSC17K0757. This work was partially supported by JSPS KAKENHI Grant Number JP16H06342, JP18H04580, and Sakigake 2018 Project of Kanazawa University.

[Computers used in this research]

This research utilized the NAOJ supercomputer ATERUI (Cray XC30) for hydrodynamic simulations of the relativistic jet. Part of the radiation transfer calculations inside the jet for light assumed to be radiated from the photosphere was performed on both ATERUI and ATERUI II (Cray XC50). ATERUI operated with a theoretical peak performance of 1.058 Pflops (petaflops) until March 2018 at NAOJ Mizusawa Campus (Oshu, Iwate). Its successor, ATERUI II, started operation boasting a theoretical peak performance of 3.087 Pflops in June 2018, also at Mizusawa Campus. (Image Credit: NAOJ)

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