GRB 180720B
The First Gamma-Ray Burst Detected in the Very-High-Energy Band

December 2019

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After a decade-long search, the H.E.S.S. collaboration has succeeded to detected a gamma-ray burst in very-high-energy gamma light for the first time. This discovery was made in July 2018 using the huge 28-m telescope (known to the community as CT5) of the H.E.S.S. array in Namibia. Surprisingly, this gamma-ray burst, an extremely energetic flash following a cosmological cataclysm, was found to emit very-high-energy gamma-rays long after the initial explosion.

Extremely energetic cosmic explosions generate gamma-ray bursts (GRB) (see e.g. [1] for a review), typically lasting for only a few tens of seconds. They are the most luminous explosions in the universe (Figure 1). The burst is followed by a longer lasting afterglow mostly in the optical and X-ray spectral regions whose intensity decreases rapidly. The prompt high energy gamma-ray emission is mostly composed of photons several MeV (Mega Electron-Volts) and can only be observed by satellite-based instruments. Whilst these spaceborne observatories have detected a few photons with even higher energies (see, e.g., [2]), the question if very-high-energy (VHE) gamma radiation (at least 100 GeV (Giga Electron-Volt) and only detectable with ground-based telescopes) is emitted, has remained unanswered until GRB 180720B was detected with the H.E.S.S. telescopes [3].

Fig. 1: Gamma-ray bursts are the most luminous explosions in the universe. Within a few seconds they radiate more energy than the sun in billions of years. Understanding the physical processes at work in these monstrous explosions are an important goal of modern astrophysics. Artist’s view of a GRB and the formation of extremely fast jets. (Credit: ESO/A. Roquette).

On 20 July 2018, the Fermi Gamma-ray Burst Monitor (GBM, [4]) and a few seconds later the Swift Burst Alert Telescope (BAT, [5]) notified the world of a gamma-ray burst, GRB 180720B. Immediately after the alert, several observatories turned to look at this position in the sky. In particular, observations with the ESO Very Large Telescope (VLT) were successful in determining a redshift of z = 0.653 ([6]). For H.E.S.S., the location of the burst in the sky became visible only 10 hours later. Nevertheless, the H.E.S.S. GRB team decided to search for a very-high-energy afterglow of the burst. After having looked for a very-high-energy signature of these events for more than a decade, the efforts by the collaboration now bore fruit ([3]).

A signature has now been detected with the large H.E.S.S. telescope that is especially suited for such observations. The data collected during two hours from 10 to 12 hours after the gamma-ray burst showed a new point-like gamma-ray source at the position of the burst (Figure 2). While the detection of GRBs at these very-high-energies had long been anticipated, the discovery many hours after the initial event, deep in the afterglow phase, came as a real surprise.

Fig. 2: GRB 180720B in very-high-energy gamma light, 10 to 12 hours after the burst as seen by the large H.E.S.S. telescope. The red cross indicates the position of GRB 180720B, determined from the optical emission of the GRB. Image taken from [3].

GRB 180720B was very strong and lasted for about 50 seconds – a relatively long duration indicating the death of a massive star. In this process, its core collapses to a rapidly rotating black hole. The surrounding gas forms an accretion disk around the black hole, with gas jets ejected perpendicularly to the disk plane creating the gamma-ray flashes. Elementary particles are accelerated in these jets to velocities nearly as high as the speed of light and interact with the surrounding matter and radiation, leading to the copious production of gamma-rays. The prompt emission phase of GRB 180720B was extremely bright, ranking seventh in brightness among the over 2650 GRBs detected by the Fermi-GBM up to date. In the high-energy (HE) gamma-ray band (100MeV–100 GeV) this GRB was also detected by the Fermi Large Area Telescope (LAT) between T0 and T0 + 700 s with a maximum photon energy of 5 GeV at T0 + 142 s [7]. No further HE emission was detected in the successive observation windows after 700 s.

The very-high-energy gamma radiation which has now been detected not only demonstrates the presence of extremely accelerated particles in GRBs, but also shows that these particles still exist or are created a long time after the explosion. Most probably, the shock wave of the explosion acts here as the cosmic accelerator. Before this H.E.S.S. observation, it had been assumed that such bursts likely are observable only within the first seconds and minutes at these extreme energies, and not many hours after the explosion.

At the time of the H.E.S.S. measurements, the X-ray afterglow had already decayed very considerably. Remarkably, the intensities (energy flux) and spectral shapes are similar in the X-ray and gamma-ray regions both at early times by Fermi-LAT and Swift-XRT and at late times by H.E.S.S. and Swift-XRT. (Figure 3). While there are several mechanisms to explain the generation of very-high-energy gamma light by particles accelerated to very-high-energies. The H.E.S.S results strongly constrain the emission to two potential mechanisms: Synchrotron emission and Synchrotron-Self-Compton Emission of electrons. Synchrotron-Self-Compton emission is preferred on energetic grounds. In both cases, however, the hardness of the H.E.S.S. spectrum, and the energy range of the emission at such late times present a major challenge.

This breakthrough discovery provides new insights into the nature of gamma-ray bursts. The detection at very late stages has already revolutionised the way we search for GRBs with Cherenkov Telescopes. Thanks to this GRB and the lessons learnt, the recently improved observational strategy of GRBs has already payed off.

Fig. 3: The multi-wavelength light curve of GRB 180720B: The upper panel a shows the energy flux light curve detected by the Fermi-GBM (Band fit, green), Fermi-LAT (power-law, blue), H.E.S.S energy flux and 95% Cl L.(power-law intrinsic, red), optical r-band (purple). The Swift-BAT spectra (15 keV–150 keV) extrapolated to the XRT band (0.3–10 keV) for a combined light curve (grey). The black dashed line indicates a temporal decay with a power-law slope of -1.2. The lower panel b shows the photon index of the Fermi-LAT, Swift and H.E.S.S. spectra. Error bars correspond to one standard deviation. Image taken from [3].


[1] P. Meszaros, "Gamma-ray bursts", Reports on Progress in Physics, 69, 2259–2321 (2006)
[2] M. Ackermann et al., "Fermi-LAT Observations of the Gamma-Ray Burst GRB 130427A", Science 343, 42–47 (2014)
[3] H.E.S.S. collaboration (H. Abdalla et al.), "A very-high-energy component deep in the Gamma-ray Burst afterglow", Nature, 575, 464–467, (2019)
[4] O.J. Roberts et al., "GRB 180720B: Fermi-GBM observation", GCN Circulars 22981 (2018)
[5] M.H. Siegel et al., "GRB 180720B: Swift detection of a burst", GCN Circulars 22973 (2018)
[6] D. Malesani et al., "VLT/X-shooter redshift", GCN Circulars 22996 (2018)
[7] E. Bissaldi et al., "GRB 180720B: Fermi-LAT detection", GCN Circulars 22980 (2018)