Gamma rays from a reverse shock with turbulent magnetic fields in GRB 180720B

Makoto Arimoto, Katsuaki Asano, Koji S. Kawabata, Kenji Toma, Ramandeep Gill, Jonathan Granot, Masanori Ohno, Shuta Takahashi, Naoki Ogino, Hatsune Goto, Kengo Nakamura, Tatsuya Nakaoka, Kengo Takagi, Miho Kawabata, Masayuki Yamanaka, Mahito Sasada, Soebur Razzaque

Research output: Contribution to journalArticlepeer-review


Gamma-ray bursts (GRBs) are the most electromagnetically luminous cosmic explosions. They are powered by collimated streams of plasma (jets) ejected by a newborn stellar-mass black hole or neutron star at relativistic velocities. Their short-lived (typically tens of seconds) prompt γ-ray emission from within the ejecta is followed by long-lived multi-wavelength afterglow emission from the ultra-relativistic forward shock. This shock is driven into the circumburst medium by the GRB ejecta. which are in turn decelerated by a mildly relativistic reverse shock. Forward-shock emission was recently detected as teraelectronvolt-energy γ-rays. Such very-high-energy emission was also predicted from the reverse shock. Here we report the detection of optical and gigaelectronvolt-energy γ-ray emission from GRB 180720B during the first few hundred seconds, which is explained by synchrotron and inverse-Compton emission from the reverse shock propagating into the ejecta, implying a low-magnetization ejecta. Our optical measurements show a clear transition from the reverse shock to the forward shock driven into the circumburst medium, accompanied by a 90° change in the mean polarization angle and fluctuations in the polarization degree and angle. This indicates turbulence with large-scale toroidal and radially stretched magnetic-field structures in the reverse and forward shocks, respectively, which tightly couple to the physics of relativistic shocks and GRB jets, namely launching, composition, dissipation and particle acceleration.

Original languageEnglish
Pages (from-to)134-144
Number of pages11
JournalNature Astronomy
Issue number1
Publication statusPublished - Jan 2024

ASJC Scopus subject areas

  • Astronomy and Astrophysics


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