Revisiting GRB 060218: new insights into low-luminosity gamma-ray bursts from a revised shock breakout model

TL;DR

This paper revisits the shock breakout model for low-luminosity gamma-ray bursts, proposing a new scenario where non-thermal equilibrium effects explain observed spectral features and rapid evolution, supporting a shock breakout origin for these events.

Contribution

It introduces a revised shock breakout model accounting for non-thermal equilibrium and light travel effects, explaining peculiar spectral features and energy requirements in low-luminosity GRBs.

Findings

Explains strong optical emission and spectral components via non-thermal equilibrium effects.

Reproduces rapid peak energy decay through thermalization of free-free emission.

Suggests >10^50 erg energy deposition, possibly from a choked jet.

Abstract

Despite two decades since the discovery of low-luminosity gamma-ray bursts, their origin remains poorly understood. In events such as GRB 060218, shock breakout from a progenitor with an extended ($10^{13}$ - $10^{14}$ cm), low-mass (0.01 - 0.1 M$_\odot$) envelope provides one possible interpretation for the smooth prompt X-ray emission lasting $\sim 1000$ s and the early optical peak at $\sim 0.5$ d. However, current shock breakout models have difficulties explaining the unexpectedly strong optical emission at $\sim 100$ s, the simultaneous presence of blackbody and power-law components in the X-ray spectrum, and the rapid evolution of the peak energy. We suggest that these peculiar features can be explained by a recently realized shock breakout scenario, in which the gas and the radiation are initially out of thermal equilibrium, but they achieve equilibrium on a time-scale faster than the light-crossing time of the envelope. In this non-standard case, due to the effects of light travel time, the observed X-ray spectrum is a multi-temperature blend of blackbody and free-free components. The free-free emission is spectrally broad, peaking in hard X-rays while also enhancing the early optical signal. As the system thermalizes, the free-free component quickly evolves toward lower energies, reproducing the observed rapid peak energy decay. To match observations, we find that more than $10^{50}$ erg must be deposited in the envelope, which may be accomplished by a choked jet. These results strengthen the case for a shock breakout origin of $ll$GRBs, and provide further evidence connecting $ll$GRBs to peculiar progenitors with extended low-mass envelopes.