GRB 110530A: Peculiar Broad Bump and Delayed Plateau in Early Optical Afterglows

TL;DR

This paper presents early optical and X-ray observations of GRB 110530A, revealing a peculiar broad bump and plateau in the afterglow lightcurve, modeled with a standard external shock including delayed energy injection, suggesting a matter-dominated jet.

Contribution

It provides the first detailed analysis of early optical afterglow features of GRB 110530A, incorporating a combined optical and X-ray fit with delayed energy injection in the external shock model.

Findings

Optical and X-ray afterglows are consistent with a single regime.

The jet has a low radiative efficiency (~1%).

The jet is likely matter dominated with very low magnetic energy fraction.

Abstract

We report our very early optical observations of GRB 110530A and investigate its jet properties together with its X-ray afterglow data. A peculiar broad onset bump followed by a plateau is observed in its early R band afterglow lightcurve. The optical data in the other bands and the X-ray data are well consistent with the temporal feature of the R band lightcurve. Our joint spectral fits of the optical and X-ray data show that they are in the same regime, with a photon index of $\sim 1.70$. The optical and X-ray afterglow lightcurves are well fitted with the standard external shock model by considering a delayed energy injection component. Based on our modeling results, we find that the radiative efficiency of the GRB jet is $\sim 1\%$ and the magnetization parameter of the afterglow jet is $<0.04$ with the derived extremely low $\epsilon_B$ (the fraction of shock energy to magnetic field) of $(1.64\pm 0.25)\times 10^{-6}$. These results indicate that the jet may be matter dominated. Discussion on delayed energy injection from accretion of late fall-back material of its pre-supernova star is also presented.