The shallow-decay phase in both optical and x-ray afterglows of Swift GRB 090529A: Energy injection into a wind-type medium?

TL;DR

This study presents optical and X-ray observations of GRB 090529A, analyzing the shallow-decay phase and suggesting energy injection into a wind-type medium, with implications for understanding GRB afterglow physics.

Contribution

It provides simultaneous optical and X-ray data of GRB 090529A, exploring the nature of the shallow-decay phase and testing energy injection models in different media.

Findings

Optical and X-ray afterglows show achromatic decay with a break at ~10^5 sec.

The post-break decay is consistent with a forward shock in an ISM medium without energy injection.

The shallow-decay phase may be due to energy injection in a wind-like medium, but other models are also possible.

Abstract

The energy injection model is usually proposed to interpret the shallow-decay phase in Swift GRB X-ray afterglows. However, very few GRBs have simultaneous signatures of energy injection in their optical and X-ray afterglows. Here, we report optical observations of GRB 090529A from 2000 sec to $\sim10^6$ sec after the burst, in which an achromatic decay is seen at both wavelengths. The optical light curve shows a decay from 0.37 to 0.99 with a break at $\sim10^5$ sec. In the same time interval, the decay indices of the X-ray light curve changed from 0.04 to 1.2. Comparing these values with the closure relations, the segment after 3$\times10^{4}$ sec is consistent with the prediction of the forward shock in an ISM medium without any energy injection. The shallow-decay phase between 2000 to 3$\times10^{4}$ sec could be due to the external shock in a wind-type-like medium with an energy injection under the condition of $\nu_o < \nu_c < \nu_x$. However, the constraint of the spectral region is not well consistent with the multi-band observations. For this shallow-decay phase, other models are also possible, such as energy injection with evolving microphysical parameters, or a jet viewed off-axis,etc.