Gamma-ray bursts

On 20 July 2018 H.E.S.S. followed up a burst detected by Swift, Fermi and other satellites. About ten hours after the burst, two hours of data taken with the large 28 m H.E.S.S. II telescope revealed a new point-like source at the burst position: the first detection of very-high-energy gamma rays from a gamma-ray burst, deep in its afterglow. The result was published in Nature in November 2019, together with the MAGIC detection of GRB 190114C.

After GRB 180720B proved that very-high-energy radiation exists in gamma-ray burst afterglows, the H.E.S.S. observations of GRB 190829A that I led brought more surprises. At z = 0.0785 it is one of the nearest bursts known. H.E.S.S. started observing about 4.5 hours after the burst and followed it for several nights. The emission turned out to be present in a low-luminosity burst too, it lasted for days, and the X-ray and TeV spectra look strikingly alike, which is hard to explain with current models.

Radiation from the brightest gamma-ray burst ever recorded was seen by practically every X-ray detector. Bright moonlight meant two long days of waiting before H.E.S.S. could start its campaign, which then continued for more than a week. The data show no sign of very-high-energy photons from this monster burst, but the limits on the gamma-ray flux still guide the theoretical understanding of this exceptional event. Its radio afterglow was followed for 475 days; see Radio transients.

The second H.E.S.S. gamma-ray burst catalogue, published in 2026, is a full re-analysis of fifteen years of follow-up observations: more than 100 bursts observed at very high energies. The work was led by Mathieu de Bony, a MOTS postdoc at IRFU. Comparing the H.E.S.S. data with Fermi and Swift observations, it shows that bursts need to be both very bright and relatively nearby to be seen at very high energies, defines the horizon set by absorption on the extragalactic background light, and supports very-high-energy emission as a common feature of afterglows.