What can we learn from "internal plateaus"? The peculiar afterglow of GRB 070110.

Context. The origin of the prompt emission of gamma-ray bursts is highly debated. Proposed scenarios involve various dissipation processes (shocks, magnetic reconnection, and inelastic collisions) above or below the photosphere of an ultra-relativistic outflow. Aims. We search for observational features that could help to favour one scenario over the others by constraining the dissipation radius, the magnetization of the outflow, or by indicating the presence of shocks. Bursts showing peculiar behaviours can emphasize the role of a specific physical ingredient, which becomes more apparent under certain circumstances. Methods. We study GRB 070110, which exhibited several remarkable features during its early afterglow; i.e. a very flat plateau terminated by an extremely steep drop and immediately followed by a bump. We modelled the plateau as the photospheric emission from a long-lasting outflow of moderate Lorentz factor (Γ ~ 20), which lags behind an ultra-relativistic (Γ > 100) ejecta that is responsible for the prompt emission. We computed the dissipation of energy in the forward and reverse shocks resulting from the deceleration of this ejecta by the external medium (uniform or stellar wind). Results. We find that photospheric emission from the long-lasting outflow can account for the plateau properties (luminosity and spectrum) assuming that some dissipation takes place in the flow. The geometrical timescale at the photospheric radius is so short that the observed decline at the end of the plateau likely corresponds to the actual shutdown of the activity in the central engine. The bump that follows results from the power dissipated in the reverse shock, which develops when the material making the plateau catches up with the initially fast shell in front, after the fast shell has decelerated. Conclusions. The proposed interpretation suggests that the prompt phase results from dissipation above the photosphere while the plateau has a photospheric origin. If the bump is produced by the reverse shock, it implies an upper limit (σ ≲ 0.1) on the magnetization of the low Γ material making the plateau. A plateau that is terminated by a drop as steep as in GRB 070110 was not observed in any other long burst. It could mean that persistent outflows are very uncommon or that the plateau luminosity or the energy of the emitted photons are generally much lower because the outflow remains mostly adiabatic or has a Lorentz factor below 10. [ABSTRACT FROM AUTHOR]