  Gamma-ray bursts (GRBs) are brief flashes of gamma rays, considered to be the
most energetic explosive phenomena in the Universe. 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
observed. GRBs typically emit most of their energy via gamma-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
instruments. 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 elussive. 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 gamma-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.

High Energy Astrophysical Phenomena