Shear Particle Acceleration in Structured Gamma-Ray Burst Jets: III. The Radiation Physics of Bright Prompt Optical Flash

TL;DR

This paper models the bright optical flashes in gamma-ray bursts as synchrotron radiation from shear-accelerated electrons in a structured jet-cocoon system, explaining observed spectral features and suggesting highly magnetized jets.

Contribution

It introduces a shear acceleration-based model for optical flashes in GRBs with a structured jet and cocoon, linking magnetic field strength to observed emissions.

Findings

The model reproduces optical flashes of GRBs 990123, 080319B, and 130427A.

Synchrotron radiation from shear-accelerated electrons explains the optical brightness.

High magnetic fields (>10^5 G) imply highly magnetized GRB jets.

Abstract

The radiation physics of bright prompt optical emission of gamma-ray bursts (GRBs) remains a puzzle. Assuming that the GRB ejecta is structured, we investigated this issue by characterizing the ejecta as an ultra-relativistic uniform jet core surrounded by a mild-relativistic cocoon. The mixed jet-cocoon (MJC) region can accelerate particles through the shear acceleration mechanism. Parameterizing the radial velocity profile of the MJC region with an exponential function and assuming a uniform magnetic field configuration, we show that the synchrotron radiation of the shear-accelerated electrons can produce a bright optical flash. Emission of the self-synchrotron Compton (SSC) process of the electron population can result in an X-ray excess and an extra MeV-GeV gamma-ray flash relative to the Band function component in the keV-MeV band, which is attributed to the synchrotron radiation of the shock-accelerated electrons in the jet core. Our model reasonably represents the extremely bright optical flash and spectral characteristics of GRBs 990123, 080319B, and 130427A. The inferred magnetic field strength of the MJC region is above $10^{5}$ G, potentially suggesting that the jets of these GRBs are highly magnetized.