Optical Behavior of GRB 061121 around its X-Ray Shallow Decay Phase

TL;DR

This study analyzes the optical and X-ray afterglows of GRB 061121, revealing a complex optical light curve with a hump and suggesting distinct emission regions, challenging standard models and indicating possible evolving shock microphysics or multiple shock components.

Contribution

It provides detailed optical observations of GRB 061121's afterglow, identifying a hump in the optical light curve and proposing new interpretations for the emission mechanisms.

Findings

Optical afterglow showed a break similar to X-ray but with a hump before the break.

The optical hump was absent in X-ray light curve.

Standard synchrotron-shock model does not fully explain the observations.

Abstract

Aims. We report on a detailed study of the optical afterglow of GRB 061121 with our original time-series photometric data. In conjunction with X-ray observations, we discuss the origin of its optical and X-ray afterglows. Methods. We observed the optical afterglow of Swift burst GRB 061121 with the Kanata 1.5-m telescope at Higashi-Hiroshima Observatory. Our observation covers a period just after an X-ray plateau phase. We also performed deep imaging with the Subaru telescope in 2010 in order to estimate the contamination of the host galaxy. Results. In the light curve, we find that the optical afterglow also exhibited a break as in the X-ray afterglow. However, our observation suggests a possible hump structure or a flattening period before the optical break in the light curve. There is no sign of such a hump in the X-ray light curve. Conclusions. This implies that the emitting region of optical was distinct from that of X-rays. The hump in the optical light curve was possibly caused by the passage of the typical frequency of synchrotron emission from another forward shock distinct from the early afterglow. The observed decay and spectral indices are inconsistent with the standard synchrotron-shock model. Hence, the observation requires a change in microphysical parameters in the shock region or a prior activity of the central engine. Alternatively, the emission during the shallow decay phase may be a composition of two forward shock emissions, as indicated by the hump structure in the light curve.