Exceptionally bright optical emission from a rare and distant γ−ray burst
Long γ-ray bursts (GRBs) are produced by the dissipation of ultra-relativistic jets launched by newly-born black holes after a collapse of massive stars. Right after the luminous and highly variable γ-ray emission, the multi-wavelength afterglow is released by the external dissipation of the jet in circumburst medium. We report the discovery of very bright (∼10 mag) optical emission ∼28 s after the explosion of the extremely luminous and energetic GRB 210619B located at redshift 1.937. Early multi-filter observations allowed us to witness the end of the shock wave propagation into the GRB ejecta. We observed the spectral transition from a bright reverse to the forward shock emission, demonstrating that the GRB multi-wavelength emission is originated from a narrow and highly magnetised jet propagating into a rarefied interstellar medium. We also find evidence of an additional component of radiation, coming from the jet wings which is able explain the uncorrelated optical/X-ray emission.