The shallow‐decay phase in both the optical and X‐ray afterglows of Swift GRB 090529A: energy injection into a wind‐type medium?

Abstract

The energy injection model is the usual choice for interpreting the shallow‐decay phase in Swift gamma‐ray burst (GRB) X‐ray afterglows. However, very few GRBs have simultaneous signatures of energy injection in their optical and X‐ray afterglows. Here, we report on the optical observations of GRB 090529A from 2000 s to ∼10 6 s 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 ∼10 5 s. 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 × 10 4 s is consistent with the prediction of the forward shock in an interstellar medium without any energy injection. The shallow‐decay phase between 2000 and 3 × 10 4 s could be a result of the external shock in a wind‐type medium with an energy injection under the condition of ν o < ν c < ν x . However, the constraint of the spectral region is not consistent with the multiband observations. For this shallow‐decay phase, other models are also possible, such as energy injection with evolving microphysical parameters, a jet viewed off‐axis, etc.