2025MNRAS.542.1269I

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.