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

Nature Astronomy volume 8, pages 134–144 (2024)Cite this article

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Abstract

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.

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Fig. 1: Light curves from the radio to teraelectronvolt bands of GRB 180720B.
Fig. 2: Optical lights curve, PDs and PAs of GRB 180720B and nearby stars.
Fig. 3: SEDs from T0 + 80 s to T0 + 300 s and theoretical modelling for time interval II.
Fig. 4: Our polarization model.

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Data availability

The Fermi-LAT data are publicly available at the Fermi Science Support Center website: https://fermi.gsfc.nasa.gov/ssc/. Swift-XRT and BAT products are available from the online GRB repository https://www.swift.ac.uk/xrt_products. All the raw data from HOWPol and HONIR can be downloaded from the SMOKA data archiving site within the website of the National Astronomical Observatory of Japan: https://smoka.nao.ac.jp/index.jsp. The processed data are available from the corresponding author upon request.

Code availability

The details of the code are fully described in Methods. Code that can reproduce each figure in the paper is available from the corresponding author upon request.

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Acknowledgements

The Fermi-LAT Collaboration acknowledges support for LAT development, operation and data analysis from NASA and the Department of the Energy (DOE) (United States); the Institute of Research into the Fundamental Laws of the Universe within the French Alternative Energies and Atomic Energy Commission and Institut national de physique nucléaire et de physique des particules within the French National Centre for Scientific Research (France); the Italian Space Agency and Istituto Nazionale di Fisica Nucleare (Italy); the Ministry of Education, Culture, Sports, Science and Technology (MEXT), the High Energy Accelerator Research Organization and the Japan Aerospace Exploration Agency (Japan); and the K.A. Wallenberg Foundation, the Swedish Research Council and the National Space Board (Sweden). Science analysis support in the operations phase from the National Institute for Astrophysics (Italy) and the National Centre for Space Studies (France) is also gratefully acknowledged. This work was performed in part under DOE Contract DE-AC02-76SF00515 and was supported by MEXT and the Japan Society for the Promotion of Science (JSPS) (KAKENHI Grant Nos. JP17H06362 and JP23H04898), the JSPS Leading Initiative for Excellent Young Researchers programme and the CHOZEN Project of Kanazawa University (M.A.). R.G. acknowledges financial support from the UNAM-DGAPA-PAPIIT IA105823 grant (Mexico). J.G. acknowledges financial support from a joint research programme of the Natural Science Foundation of China and the Israel Science Foundation (Grant No. 3296/19).

Author information

Authors and Affiliations

  1. Institute of Science and Engineering, Kanazawa University, Kanazawa, Japan

    Makoto Arimoto, Shuta Takahashi, Naoki Ogino & Hatsune Goto

  2. Institute for Cosmic Ray Research, The University of Tokyo, Kashiwa, Japan

    Katsuaki Asano

  3. Hiroshima Astrophysical Science Center, Hiroshima University, Higashi-Hiroshima, Japan

    Koji S. Kawabata, Masanori Ohno, Tatsuya Nakaoka, Masayuki Yamanaka & Mahito Sasada

  4. Department of Physical Science, Hiroshima University, Higashi-Hiroshima, Japan

    Koji S. Kawabata, Masanori Ohno, Kengo Nakamura, Tatsuya Nakaoka, Kengo Takagi & Miho Kawabata

  5. Frontier Research Institute for Interdisciplinary Sciences, Tohoku University, Sendai, Japan

    Kenji Toma

  6. Graduate School of Science, Tohoku University, Sendai, Japan

    Kenji Toma

  7. Instituto de Radioastronomía y Astrofísica, Universidad Nacional Autónoma de México, Morelia, Mexico

    Ramandeep Gill

  8. Astrophysics Research Center of the Open university (ARCO), The Open University of Israel, Ra’anana, Israel

    Ramandeep Gill & Jonathan Granot

  9. Department of Natural Sciences, The Open University of Israel, Ra’anana, Israel

    Jonathan Granot

  10. Department of Physics, The George Washington University, Washington, DC, USA

    Jonathan Granot & Soebur Razzaque

  11. Nishi-Harima Astronomical Observatory, Center for Astronomy, University of Hyogo, Sayo, Japan

    Miho Kawabata

  12. Amanogawa Galaxy Astronomy Research Center, Kagoshima University, Kagoshima, Japan

    Masayuki Yamanaka

  13. Department of Physics, Tokyo Institute of Technology, Tokyo, Japan

    Mahito Sasada

  14. Centre for Astro-Particle Physics (CAPP) and Department of Physics, University of Johannesburg, Auckland Park, South Africa

    Soebur Razzaque

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  1. Makoto Arimoto
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Contributions

M.A. contributed to the analysis of the X-ray and gigaelectronvolt data, the interpretation and the writing of the manuscript. K.A., K. Toma, R.G. and J.G. provided the interpretation and contributed to the writing of the paper. S.R. contributed to the interpretation of the GRB model. K.K., K.N., T.N., K. Takagi, M.K., M.Y. and M.S. contributed to the optical Kanata observations and the optical data analysis. M.O., S.T., N.O. and H.G. analysed the X-ray and gigaelectronvolt data. All authors reviewed the manuscript.

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Correspondence to Makoto Arimoto.

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Extended data

Extended Data Fig. 1 Lightcurves of the afterglow with the analytical model.

The observed flux density lightcurves at different frequencies (Fermi-LAT at 300 MeV, HESS at 300 GeV, optical at 4.6 × 1014 Hz, Swift-XRT at 2 keV, Swift-BAT at 30 keV, and radio at 15.5 GHz) are shown along with the theoretical reverse-shock (dotted), forward-shock (dashed) and combined reverse-shock plus forward-shock (solid) components. Note that the reverse-shock emission in the XRT band is suppressed because the maximum synchrotron frequency is much lower than the X-ray band. Errors correspond to the 1-σ confidence region.

Extended Data Fig. 2 Theoretical model with time-independent parameters at time intervals II and III.

(a) Spectral energy distribution at time interval II with the EATS model. The reverse- (RS) and forward-shock (FS) components are shown with the synchrotron and SSC emission. (b) Spectral energy distribution at time interval III with the EATS model. The legend shows the adopted model parameters. Here Γ0 is the bulk Lorentz factor of the coasting flow before it is decelerated by the ISM, Einj is the amount of energy injected during the shallow plateau phase, and subscripts ‘f’ and ‘r’ refer to FS and RS parameters, respectively. (c) Multi-waveband lightcurve and model comparison. The vertical line shows the duration of the prompt GRB. See the caption of Extended Data Fig. 1 for details. Errors correspond to the 1-σ confidence region.

Extended Data Fig. 3 Spectral energy distribution at time interval III.

The solid lines in the low-energy and high-energy bands represent the synchrotron and SSC components from the forward shock with the “analytical” model, respectively. The red area corresponds to the 1-σ confidence region from the best-fit power-law function for the Swift-XRT. Note that the XRT observation was not actually performed in the time interval and we used the interpolated flux before and after the interval (this interpolation is reasonable because the photon index is almost constant from T0 + 104 s to 105 s, as shown in the bottom panel of Fig. 1. The blue arrow represents the 90% upper limit in the Fermi-LAT range. The red point represents the optical flux observed by the optical telescope. The purple area represents the 1-σ confidence region from the best-fit power-law function for the HESS.

Extended Data Table 1 Model parameters used for reverse and forward shocks afterglow modeling and output parameters
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Supplementary information

Supplementary Information

Supplementary Methods, Figs. 1–6 and Tables 1 and 2.

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Arimoto, M., Asano, K., Kawabata, K.S. et al. Gamma rays from a reverse shock with turbulent magnetic fields in GRB 180720B. Nat Astron 8, 134–144 (2024). https://doi.org/10.1038/s41550-023-02119-1

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