Insights on the origin of low-luminosity GRBs from a revised shock breakout model for GRB 060218

Despite ~two decades since the discovery of low-luminosity gamma-ray bursts (LLGRBs), their origin remains poorly understood. Shock breakout from a progenitor with an extended (~10¹³ cm), low-mass (10^-2 Msun) envelope provides a possible interpretation for the smooth prompt X-ray light curve lasting ~1000 s and the early optical peak at ~0.5 days observed in some events. However, even in the best-observed case, GRB 060218, current shock breakout models have difficulties explaining the unexpectedly strong optical emission at a few hundred seconds, the simultaneous presence of thermal and non-thermal 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 previously unexplored regime of shock breakout. Applying recent advances in shock breakout theory which predict more rapid thermalization in the early planar phase of evolution, we show that for sufficiently fast shocks breaking out of especially extended progenitors, a non-standard breakout scenario is expected in which the gas and radiation are initially out of thermal equilibrium, but the time to reach equilibrium is less than the light-crossing time of the envelope. In this case, due to light travel time effects, the observed X-ray spectrum is a multi-temperature blend of blackbody and free-free emission components. The bremsstrahlung component extends down to the optical band, which can account for the excess optical emission observed at early times. As the system thermalizes, the non-thermal component quickly evolves toward lower energies and eventually fades altogether, resulting in a rapid peak energy decay consistent with observations. These results strengthen the case for a shock breakout origin of LLGRBs, and provide further evidence connecting LLGRBs to peculiar progenitors with extended low-mass envelopes.