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.
Subscribe to Journal
Get full journal access for 1 year
only $3.90 per issue
All prices are NET prices.
VAT will be added later in the checkout.
Rent or Buy article
Get time limited or full article access on ReadCube.
All prices are NET prices.
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/).
Mészáros, P. Gamma-ray bursts. Rep. Prog. Phys. 69, 2259–2321 (2006).
Zhang, B. & Mészáros, P. Gamma-ray bursts: progress, problems & prospects. Int. J. Mod. Phys. A 19, 2385–2472 (2004).
Ackermann, M. et al. Fermi-LAT observations of the gamma-ray burst GRB 130427A. Science 343, 42–47 (2014).
Piron, F. Gamma-ray bursts at high and very high energies. C. R. Phys. 17, 617–631 (2016).
Roberts, O. J. et al. GCN22981 – GRB 180720B: Fermi-GBM observation. GCN Circulars https://gcn.gsfc.nasa.gov/gcn3/22981.gcn3 (2018).
Siegel, M. H. et al. GCN22973 – GRB 180720B: Swift detection of a burst. GCN Circulars https://gcn.gsfc.nasa.gov/gcn3/22973.gcn3 (2018).
Malesani, D. et al. GCN22996 – VLT/X-shooter redshift. GCN Circulars https://gcn.gsfc.nasa.gov/gcn3/22996.gcn3 (2018).
Bissaldi, E. et al. GCN22980 – GRB 180720B: Fermi-LAT detection. GCN Circulars https://gcn.gsfc.nasa.gov/gcn3/22980.gcn3 (2018).
Levan, A. et al. Gamma-ray burst progenitors. Space Sci. Rev. 202, 33–78 (2016).
Evans, P. A. et al. Methods and results of an automatic analysis of a complete sample of Swift-XRT observations of GRBs. Mon. Not. R. Astron. Soc. 397, 1177–1201 (2009).
Evans, P. A. et al. An online repository of Swift/XRT light curves of γ-ray bursts. Astron. Astrophys. 469, 379–385 (2007).
Schmalz, S. et al. GCN23020 – ISON-Castelgrande observation of GRB 180720B. GCN Circulars https://gcn.gsfc.nasa.gov/gcn3/23020.gcn3 (2018).
Franceschini, A., Rodighiero, G. & Vaccari, M. Extragalactic optical-infrared background radiation, its time evolution and the cosmic photon-photon opacity. Astron. Astrophys. 487, 837–852 (2008).
Ajello, M. et al. A decade of gamma-ray bursts observed by Fermi-LAT: the second GRB catalog. Astrophys. J. 878, 52 (2019).
Chevalier, R. A. & Li, Z. Y. Wind interaction models for gamma-ray burst afterglows: the case for two types of progenitors. Astrophys. J. 536, 195–212 (2000).
Kumar, P. & Barnoil Duran, R. On the generation of high-energy photons detected by the Fermi Satellite from gamma-ray bursts. Mon. Not. R. Astron. Soc. 400, L75–L79 (2009).
Sari, R. & Esin, A. A. On the synchrotron self-Compton emission from relativistic shocks and its implications for gamma-ray burst afterglows. Astrophys. J. 548, 787–799 (2001).
Zhang, B. & Mészáros, P. High-energy spectral components in gamma-ray burst afterglows. Astrophys. J. 559, 110–122 (2001).
Warren, D. C. et al. Nonlinear particle acceleration and thermal particles in GRB afterglows. Astrophys. J. 835, 248 (2017).
Guilbert, P. W., Fabian, A. C. & Rees, M. J. Spectral and variability constraints on compact sources. Mon. Not. R. Astron. Soc. 205, 593–603 (1983).
Aharonian, F. A., Belyanin, A. A., Derishev, E. V., Kocharovsky, V. V. & Kocharovsky, V. I. V. Constraints on the extremely high-energy cosmic ray accelerators from classical electrodynamics. Phys. Rev. D 66, 023005 (2002).
Aharonian, F. A. TeV gamma rays from BL Lac objects due to synchrotron radiation of extremely high energy protons. New Astron. 5, 377–395 (2000).
Warren, D. C., Barkov, M. V., Hirotaka, I., Nagataki, S. & Laskar, T. Synchrotron self-absorption in GRB afterglows: the effects of a thermal electron population. Mon. Not. R. Astron. Soc. 480, 4060 (2018).
Nakar, E., Ando, S. & Sari, R. Klein–Nishina effects on optically thin synchrotron and synchrotron self-Compton spectrum. Astrophys. J. 703, 675–691 (2009).
de Naurois, M. et al. GRB190829A: Detection of VHE gamma-ray emission with HESS. The Astronomer’s Telegram 13052 (2019).
Mirzoyan, R. First time detection of a GRB at sub-TeV energies; MAGIC detects the GRB 190114C. The Astronomer’s Telegram 12390 (2019).
CTA Consortium. Science with the Cherenkov Telescope Array (World Scientific Publishing, 2019).
Hofverberg, P. et al. Commissioning and initial performance of the H.E.S.S. II drive system. In Proc. of the 33rd International Cosmic Ray Conference (ICRC 2013), 3092 (Curran Associates, 2013).
Bathelmy, S. GCN: The gamma-ray burst coordinates network http:/gcn.gsfc.nasa.gov (2019).
Holler, M. et al. Observations of the Crab Nebula with H.E.S.S. Phase II. PoS Proc. Sci. ICRC2015, 847 (2016).
Berge, D., Funk, S. & Hinton, J. Background modelling in very-high-energy gamma-ray astronomy. Astron. Astrophys. 466, 1219–1229 (2007).
Parsons, R. D. & Hinton, J. A. A Monte Carlo template based analysis for air-Cherenkov arrays. Astropart. Phys. 56, 26–34 (2014).
Parsons, R. D., Murach, T. & Gajdus, M. HESS II data analysis with ImPACT. PoS Proc. Sci. ICRC2015, 826 (2015).
Murach, T., Gajdus, M. & Parsons, R. D. A neural network-based monoscopic reconstruction algorithm for H.E.S.S. II. PoS Proc. Sci. ICRC2015, 1022 (2015).
de Naurois, M. & Rolland, L. A high performance likelihood reconstruction of γ-rays for imaging atmospheric Cherenkov telescopes. Astropart. Phys. 32, 231–252 (2009).
Li, T. P. & Ma, Y. Q. Analysis methods for results in gamma-ray astronomy. Astrophys. J. 272, 317–324 (1983).
Piron, F. et al. Temporal and spectral gamma-ray properties of Mkn 421 above 250 GeV from CAT observations between 1996 and 2000. Astron. Astrophys. 374, 895–906 (2001).
Abdalla, H. et al. Gamma-ray blazar spectra with H.E.S.S. II mono analysis: the case of PKS 2155-304 and PG 1553+113. Astron. Astrophys. 600, A89 (2017).
Domínguez, A. et al. Extragalactic background light inferred from AEGIS galaxy-SED-type fractions. Mon. Not. R. Astron. Soc. 410, 2556–2578 (2011).
Finke, J. D., Razzaque, S. & Dermer, C. D. Modeling the extragalactic background light from stars and dust. Astrophys. J. 712, 238–249 (2010).
Gilmore, R. C. et al. Semi-analytic modelling of the extragalactic background light and consequences for extragalactic gamma-ray spectra. Mon. Not. R. Astron. Soc. 422, 3189–3207 (2012).
FERMIGBRST – Fermi GBM Burst Catalog https://heasarc.gsfc.nasa.gov/W3Browse/fermi/fermigbrst.html (2019).
GBM Software Tools https://fermi.gsfc.nasa.gov/ssc/data/analysis/rmfit/ (2019).
Atwood, W. et al. Pass 8: toward the full realization of the Fermi-LAT scientific potential. In 2012 Fermi Symposium proceedings, eConf C121028 (2013); preprint at https://arxiv.org/abs/1303.3514.
Fermi LAT Performance http://www.slac.stanford.edu/exp/glast/groups/canda/lat_Performance.htm (2019).
Fermitools-conda-recipe https://github.com/fermi-lat/Fermitools-conda/ (2019).
Acero, F. et al. Development of the model of galactic interstellar emission for standard point-source analysis of Fermi Large Area Telescope data. Astrophys. J. Suppl. Ser. 223, 26 (2016).
Sasada, M. et al. GCN22977 – Kanata observation. GCN Circulars https://gcn.gsfc.nasa.gov/gcn3/22977.gcn3 (2018).
Itoh, R. et al. GCN22983 – MITSuME Akeno observation. GCN Circulars https://gcn.gsfc.nasa.gov/gcn3/22983.gcn3 (2018).
Reva, I. et al. GCN22979 – TSHAO observation. GCN Circulars https://gcn.gsfc.nasa.gov/gcn3/22979.gcn3 (2018).
Lipunov, V. et al. GCN23023 – MASTER observation. GCN Circulars https://gcn.gsfc.nasa.gov/gcn3/23023.gcn3 (2018).
Kann, D. et al. GCN22985 – OSN observation. GCN Circulars https://gcn.gsfc.nasa.gov/gcn3/22985.gcn3 (2018).
Martone, R. et al. GCN22976 – LCO Haleaka observation of GRB 180720B. GCN Circulars https://gcn.gsfc.nasa.gov/gcn3/22976.gcn3 (2018).
Zheng, W. et al. GCN23033 – KAIT observation of GRB 180720B. GCN Circulars https://gcn.gsfc.nasa.gov/gcn3/23033.gcn3 (2018).
The Swift Burst Analyser – GRB 180720B https://www.swift.ac.uk/burst_analyser/00848890/ (2018).
Nousek, J. A. et al. Evidence for a canonical GRB afterglow light curve in the Swift/XRT data. Astrophys. J. 642, 389–400 (2006).
Vurm, I. & Beloborodov, A. M. On the prospects of gamma-ray burst detection in the TeV band. Astrophys. J. 846, 152 (2017).
Blandford, R. D. & McKee, C. F. Fluid dynamics of relativistic blast waves. Phys. Fluids 19, 1130–1138 (1976).
Aharonian, F. A. Very High Energy Cosmic Gamma Radiation: A Crucial Window on the Extreme Universe (World Scientific Publishing, 2004).
Blumenthal, G. R. & Gould, R. J. Bremsstrahlung, synchrotron radiation, and Compton scattering of high-energy electrons traversing dilute gases. Rev. Mod. Phys. 42, 237–270 (1970).
Kelner, S. R., Aharonian, F. A. & Khangulyan, D. On the jitter radiation. Astrophys. J. 774, 61 (2013).
Santana, R., Barniol, D. & Kumar, P. Magnetic fields in relativistic collisionless shocks. Astrophys. J. 785, 29 (2014).
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.
The authors declare no competing interests.
Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
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
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.
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.
About this article
Cite this article
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