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

• Published in: Monthly notices of the Royal Astronomical Society Vol. 422; no. 3; pp. 2044 - 2050
• Main Authors: Xin, L. P., Pozanenko, A., Kann, D. A., Xu, D., Gorosabel, J., Leloudas, G., Wei, J. Y., Andreev, M., Qin, S. F., Ibrahimov, M., Han, X. H., de Ugarte Postigo, A., Qiu, Y. L., Deng, J. S., Volnova, A., Jakobsson, P., Castro-Tirado, A. J., Aceituno, F., Fynbo, J. P. U., Wang, J., Sanchez-Ramirez, R., Kouprianov, V., Zheng, W. K., Tello, J. C., Wu, C.
• Format: Journal Article
• Language: English
• Published: Oxford, UK Blackwell Publishing Ltd 01.05.2012; Oxford University Press
• Subjects: Astrophysics; gamma‐ray burst; gamma‐ray burst: individual: GRB 090529A; Stars & galaxies; X-ray astronomy; general; gamma-ray burst: individual: GRB 090529A; gamma-ray burst
• ISSN: 0035-8711; 1365-2966
• DOI: 10.1111/j.1365-2966.2012.20681.x

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.