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

## TL;DR

This paper analyzes the peculiar afterglow of GRB 070110, proposing a model where the early plateau is due to photospheric emission from a long-lasting outflow, with the bump caused by reverse shock dissipation, providing insights into GRB emission mechanisms.

## Contribution

The study introduces a combined model of photospheric emission and shock dissipation to explain the unique features of GRB 070110's afterglow, offering new constraints on outflow magnetization.

## Key findings

- Photospheric emission can explain the plateau's luminosity and spectrum.
- The steep decline indicates shutdown of the central engine.
- The reverse shock likely causes the observed bump, constraining magnetization to σ ≲ 0.1.

## Abstract

Context: The origin of GRBs' prompt emission is highly debated. Proposed scenarios involve dissipation processes above or below the photosphere of an ultra-relativistic outflow. Aims: We search for observational features that would favour one scenario over the others by constraining the dissipation radius, the outflow magnetization or by indicating the presence of shocks. Bursts showing peculiarities 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: a very flat plateau terminated by an extremely steep drop and immediately followed by a bump. We model the plateau as photospheric emission from a long lasting outflow of moderate Lorentz factor ($\Gamma\sim 20$) which lags behind an ultra-relativistic ($\Gamma> 100$) ejecta responsible for the prompt emission. We compute the dissipation of energy in the forward and reverse shocks resulting from this ejecta's deceleration by the external medium. Results: Photospheric emission from the long-lasting outflow can account for the plateau properties (luminosity and spectrum) assuming 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 shut-down of the central engine. The following bump results from dissipated power in the reverse shock, which develops when the slower material catches up with the initially fast component, after it had been decelerated. Conclusions: Our interpretation suggests that the prompt phase resulted from dissipation above the photosphere while the plateau had a photospheric origin. If the bump is produced by the reverse shock, it implies an upper limit ($\sigma \lesssim 0.1$) on the magnetization of the slower material.