The nature of the late achromatic bump in GRB 120326A

TL;DR

This paper investigates the cause of a rare, late-time achromatic rebrightening in the optical and X-ray afterglow of GRB 120326A, ruling out several models and suggesting refreshed shocks or geometrical effects as possible explanations.

Contribution

The study provides a detailed analysis of the late achromatic bump in GRB 120326A, excluding common scenarios and proposing alternative explanations within the fireball model.

Findings

Achromatic rebrightening peaking at ~35 ks after burst

Exclusion of reverse shock, density enhancement, and microlensing scenarios

Possible explanation by refreshed shock or geometrical effect

Abstract

The long ${\it Swift}$ gamma-ray burst GRB 120326A at redshift $z=1.798$ exhibited a multi-band light curve with a striking feature: a late-time, long-lasting achromatic rebrightening, rarely seen in such events. Peaking in optical and X-ray bands $\sim 35$ ks ($\sim 12.5$ ks in the GRB rest frame) after the 70-s GRB prompt burst, the feature brightens nearly two orders of magnitude above the underlying optical power-law decay. Modelling the multiwavelength light curves, we investigate possible causes of the rebrightening in the context of the standard fireball model. We exclude a range of scenarios for the origin of this feature: reverse-shock flash, late-time forward shock peak due to the passage of the maximal synchrotron frequency through the optical band, late central engine optical/X-ray flares, interaction between the expanding blast wave and a density enhancement in the circumburst medium and gravitational microlensing. Instead we conclude that the achromatic rebrightening may be caused by a refreshed forward shock or a geometrical effect. In addition, we identify an additional component after the end of the prompt emission, that shapes the observed X-ray and optical light curves differently, ruling out a single overall emission component to explain the observed early time emission.