Exceptionally bright optical emission from a rare and distant gamma-ray burst

Long gamma-ray bursts are produced by energy dissipation within ultra-relativistic jets launched by newborn black holes after the collapse of a peculiar class of massive stars. Right after the luminous and highly variable gamma-ray emission, a multi-wavelength afterglow is released by external dissipation of the jet energy in the medium that surrounds the progenitor star. We report the discovery of a very bright (~10 mag) optical emission ~28 s after the explosion of the extremely luminous and energetic GRB 210619B located at redshift 1.937. We observed the transition from a bright reverse to the forward shock emission, demonstrating that the early and late gamma-ray-burst multi-wavelength emission originated from a narrow, magnetized jet propagating into a rarefied interstellar medium. These conditions are found to be optimal to produce the bright optical flash from the reverse shock. Slower jets propagating in denser media are expected to cause a flash of very-high-energy radiation, which is yet to be discovered.