The origin of the late rebrightening in GRB 080503

TL;DR

This paper investigates the cause of the late rebrightening in GRB 080503, comparing refreshed shocks and density clumps, and models the afterglow using both simplified and detailed methods to determine the most plausible explanation.

Contribution

It introduces detailed multizone modeling of afterglow dynamics and evaluates the effectiveness of different scenarios and models in explaining the rebrightening.

Findings

Density clump scenario is not favored.

Refreshed shocks can explain the rebrightening under certain conditions.

Multizone modeling is necessary for accurate shock dynamics description.

Abstract

GRB 080503, detected by Swift, belongs to the class of bursts whose prompt phase consists of an initial short spike followed by a longer soft tail. It did not show any transition to a regular afterglow at the end of the prompt emission but exhibited a surprising rebrightening after one day. We aim to explain this rebrightening with two different scenarios - refreshed shocks or a density clump in the circumburst medium - and two models for the origin of the afterglow, the standard one where it comes from the forward shock, and an alternative one where it results from a long-lived reverse shock. We computed afterglow light curves either using a single-zone approximation for the shocked region or a detailed multizone method that more accurately accounts for the compression of the material. We find that in several of the considered cases the detailed model must be used to obtain a reliable description of the shock dynamics. The density clump scenario is not favored. We confirm previous results that the presence of the clump has little effect on the forward shock emission, except if the microphysics parameters evolve when the shock enters the clump. Moreover, we find that the rebrightening from the reverse shock is also too weak when it is calculated with the multi-zone method. On the other hand, in the refreshed-shock scenario both the forward and reverse shock models provide satisfactory fits of the data under some additional conditions on the distribution of the Lorentz factor in the ejecta and the beaming angle of the relativistic outflow.