Optical light curve of GRB 121011A: a textbook for the onset of GRB afterglow in a mixture of ISM and wind-type medium

TL;DR

This paper presents optical and X-ray observations of GRB 121011A, demonstrating an afterglow onset consistent with a mixed ISM and wind medium, and confirms the standard external-shock model without energy injection.

Contribution

It provides a detailed analysis of the optical light curve onset in a GRB within a mixed medium, illustrating the transition from wind to ISM and supporting the standard afterglow model.

Findings

Optical light curve shows a smooth bump indicating afterglow onset.

The medium transitions from mixed ISM and wind to pure ISM.

X-ray and optical data are consistent with the standard afterglow model.

Abstract

We reported the optical observations of GRB 121011A by 0.8-m TNT telescope at Xinglong observatory, China. The light curve of optical afterglow shows a smooth and featureless bump during the epoch of $\sim$130 sec and $\sim$5000 sec with a rising index of $1.57\pm0.28$ before the break time of $539\pm44$ sec, and a decaying index of about $1.29\pm0.07$ up to the end of our observations. Meanwhile, the X-ray light curve decays in a single power-law with a slop of about $1.51\pm0.03$ observed by $XRT$ onboard ${\rm} Swift$ from 100 sec to about 10000 sec after the burst trigger. The featureless optical light curve could be understood as an onset process under the external-shock model. The typical frequency has been below or near the optical one before the deceleration time, and the cooling frequency is located between the optical and X-ray wavelengths. The external medium density has a transition from a mixed stage of ISM and wind-type medium before the peak time to the ISM at the later phase. The joint-analysis of X-ray and optical light curves shows that the emission from both frequencies are consistent with the prediction of the standard afterglow model without any energy injections, indicating that the central engine has stopped its activity and does not restart anymore after the prompt phase.