The shallow-decay phase in both the optical and X-ray afterglows of Swift GRB 090529A: energy injection into a wind-type medium?
Fynbo, J. P. U.; Kann, D. A.; Leloudas, G.; Gorosabel, J.; Castro-Tirado, A. J.; Kouprianov, V.; Jakobsson, P.; de Ugarte Postigo, A.; Wang, J.; Xin, L. P.; Deng, J. S.; Han, X. H.; Wei, J. Y.; Wu, C.; Ibrahimov, M.; Xu, D.; Pozanenko, A.; Tello, J. C.; Qiu, Y. L.; Volnova, A.; Andreev, M.; Qin, S. F.; Aceituno, F.; Sanchez-Ramirez, R.; Zheng, W. K.
China, Russia, Germany, Denmark, Spain, Ukraine, Uzbekistan, Iceland, United States
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 ∼106 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 ∼105 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 × 104 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 × 104 s could be a result of the external shock in a wind-type medium with an energy injection under the condition of νo < νc < νx. 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.