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

TL;DR

This paper reports the discovery of an exceptionally bright optical emission from a distant gamma-ray burst, revealing detailed jet dynamics and an additional radiation component, advancing understanding of GRB emission mechanisms.

Contribution

It provides the first detailed observation of the transition from reverse to forward shock emission in a highly luminous GRB, highlighting a narrow jet and rarefied medium.

Findings

Bright optical emission observed 28 seconds after GRB 210619B

Spectral transition from reverse to forward shock emission

Evidence of an additional jet wing radiation component

Abstract

Long $\rm \gamma$-ray bursts (GRBs) are produced by the dissipation of ultra-relativistic jets launched by newly-born black holes after the collapse of massive stars. Right after the luminous and highly variable $\gamma$-ray emission, the multi-wavelength afterglow is released by the external dissipation of the jet in circumburst medium. We report the discovery of a very bright ($\rm \sim 10$ mag) optical emission $\rm \sim 28$ 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 early and late GRB multi-wavelength emission is originated from a very narrow jet propagating into an unusually 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.