A very-high-energy component deep in the γ-ray burst afterglow

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Abstract

Gamma-ray bursts (GRBs) are brief flashes of γ-rays and are considered to be the most energetic explosive phenomena in the Universe1. The emission from GRBs comprises a short (typically tens of seconds) and bright prompt emission, followed by a much longer afterglow phase. During the afterglow phase, the shocked outflow—produced by the interaction between the ejected matter and the circumburst medium—slows down, and a gradual decrease in brightness is observed2. GRBs typically emit most of their energy via γ-rays with energies in the kiloelectronvolt-to-megaelectronvolt range, but a few photons with energies of tens of gigaelectronvolts have been detected by space-based instruments3. However, the origins of such high-energy (above one gigaelectronvolt) photons and the presence of very-high-energy (more than 100 gigaelectronvolts) emission have remained elusive4. Here we report observations of very-high-energy emission in the bright GRB 180720B deep in the GRB afterglow—ten hours after the end of the prompt emission phase, when the X-ray flux had already decayed by four orders of magnitude. Two possible explanations exist for the observed radiation: inverse Compton emission and synchrotron emission of ultrarelativistic electrons. Our observations show that the energy fluxes in the X-ray and γ-ray range and their photon indices remain comparable to each other throughout the afterglow. This discovery places distinct constraints on the GRB environment for both emission mechanisms, with the inverse Compton explanation alleviating the particle energy requirements for the emission observed at late times. The late timing of this detection has consequences for the future observations of GRBs at the highest energies.

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Fig. 1: Multi-wavelength light curve of GRB 180720B.
Fig. 2: Very-high-energy γ-ray image of GRB 180720B.

Data and code availability

The raw H.E.S.S. data and the code used in this study are not public, but belong to the H.E.S.S. collaboration. All derived higher-level data that are shown in the plots will be made available on the H.E.S.S. collaboration’s website upon publication of this study. Data and analysis code from the Fermi-GBM and LAT instruments are publicly available. Links to the data and software are provided in the Methods section. This work also made use of data supplied by the UK Swift Science Data Centre at the University of Leicester (http://www.swift.ac.uk/archive/).

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Acknowledgements

We acknowledge the support of the Namibian authorities and of the University of Namibia in facilitating the construction and operation of H.E.S.S. We also acknowledge support by the German Ministry for Education and Research (BMBF); the Max Planck Society; the German Research Foundation (DFG); the Helmholtz Association; the Alexander von Humboldt Foundation; the French Ministry of Higher Education, Research and Innovation; the Centre national de la recherche scientifique (CNRS/IN2P3 and CNRS/INSU); the Commissariat a l’energie atomique et aux energies alternatives (CEA); the UK Science and Technology Facilities Council (STFC); the Knut and Alice Wallenberg Foundation; the National Science Centre, Poland, through grant number 2016/22/M/ST9/00382; the South African Department of Science and Technology and National Research Foundation; the University of Namibia; the National Commission on Research, Science & Technology of Namibia (NCRST); the Austrian Federal Ministry of Education, Science and Research; the Austrian Science Fund (FWF); the Australian Research Council (ARC); the Japan Society for the Promotion of Science; and the University of Amsterdam. We appreciate the work of the technical support staff in Berlin, Zeuthen, Heidelberg, Palaiseau, Paris, Saclay, Tübingen and in Namibia for the construction and operation of the equipment. This work benefited from services provided by the H.E.S.S. Virtual Organisation, supported by the national resource providers of the EGI Federation. The Fermi-LAT Collaboration acknowledges support for LAT development, operation and data analysis from NASA, the US Department of Energy (DOE), CEA/Irfu and IN2P3/CNRS (France), ASI and INFN (Italy), MEXT, KEK, JAXA (Japan), the K. A. Wallenberg Foundation, the Swedish Research Council and the National Space Board (Sweden). Science analysis support in the operations phase from INAF (Italy) and CNES (France) is also acknowledged. This work was performed in part under DOE contract DE-AC02-76SF00515.

Author information

R.D.P., Q.P. and E.R.-V. analysed and interpreted the H.E.S.S. data and prepared the manuscript. E.B. analysed and interpreted the Fermi data and prepared the manuscript. C.H. implemented the response system for the GRBs follow-up of H.E.S.S. A.M.T., F.A. and D. Khangulyan helped to interpret the results and prepare the manuscript. The entire H.E.S.S. collaboration contributed to the publication with involvement at various stages, from the design, construction and operation of the instrument to the development and maintenance of all software for data handling, data reduction and data analysis. All authors reviewed, discussed and commented on the present results and the manuscript.

Correspondence to E. Ruiz-Velasco.

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Peer review information Nature thanks Gus Sinnis and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

Extended data figures and tables

Extended Data Fig. 1 VHE spectral fit of GRB 180720B.

H.E.S.S. spectral fit to the measured emission in the energy range 100–440 GeV. a, Fit using a simple power-law model (with photon index γobs). b, Fit with a power-law model (with photon index γint) with EBL attenuation for a source at z = 0.653 (ref. 13). In both cases the residual data points with 1σ uncertainties are obtained from the forward-folded method. The shaded areas show the statistical and systematic uncertainties in each fit (1σ confidence level). The bottom panels show the significance of the residuals between the fitted model and the data points.

Extended Data Fig. 2 EBL absorption coefficient.

Absorption coefficient eτ(E) for a source emitting at a redshift of 0.653. The values are shown in the energy range of the detected emission of GRB 180720B (100–440 GeV) for the four EBL models considered13,39,40,41.

Extended Data Fig. 3 CTA detectability prospects.

Energy-flux distribution at 11 h and 5 h after the Swift-BAT trigger for all the GRBs observed by Swift-XRT per year. The blue vertical line shows the expected sensitivity of CTA, assuming the detection of fluxes 10 times fainter than that of GRB 180720B. The energy flux of GRB 180720B is indicated by the red vertical line.

Extended Data Table 1 VHE spectral information from GRB 180720B

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Abdalla, H., Adam, R., Aharonian, F. et al. A very-high-energy component deep in the γ-ray burst afterglow. Nature 575, 464–467 (2019) doi:10.1038/s41586-019-1743-9

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