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

Despite two decades since the discovery of low-luminosity gamma-ray bursts ($ll$GRBs), 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–$0.1\, \mathrm{M}_\odot$) envelope provides one possible interpretation for the 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 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 traveltime, the observed X-ray spectrum is a multitemperature 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.