Probing the Nature of High-z Short GRB 090426 with Its Early Optical and X-ray Afterglows

Liping Xin; Enwei Liang; Jianyan Wei; Bing Zhang; Houjun Lv; Weikang Zheng; Yuji Urata; Myungshin Im; Jing Wang; Yulei Qiu; Jinsong Deng; Kuiyun Huang; Jingyao Hu; Yiseul Jeon; Huali Li; Xuhui Han

doi:10.1111/j.1365-2966.2010.17419.x

paper

Probing the Nature of High-z Short GRB 090426 with Its Early Optical and X-ray Afterglows

arXiv:1002.0889 · doi:10.1111/j.1365-2966.2010.17419.x

Abstract

GRB 090426 is a short duration burst detected by Swift ( $T_{90} \sim 1.28$ s in the observer frame, and $T_{90} \sim 0.33$ s in the burst frame at $z = 2.609$ ). Its host galaxy properties and some $γ$ -ray related correlations are analogous to those seen in long duration GRBs, which are believed to be of a massive-star origin (so-called Type II GRBs). We present the results of its early optical observations with the 0.8-m TNT telescope at Xinglong observatory, and the 1-m LOAO telescope at Mt. Lemmon Optical Astronomy Observatory in Arizona. Our well-sampled optical afterglow lightcurve covers from $\sim 90$ seconds to $\sim 1 0^{4}$ seconds post the GRB trigger. It shows two shallow decay episodes that are likely due to energy injection, which end at $\sim 230$ seconds and $\sim 7100$ seconds, respectively. The decay slopes post the injection phases are consistent with each other ( $α ≃ 1.22$ ). The X-ray afterglow lightcurve appears to trace the optical, although the second energy injection phase was missed due to visibility constraints introduced by the {\em Swift} orbit. The X-ray spectral index is $β_{X} \sim 1.0$ without temporal evolution. Its decay slope is consistent with the prediction of the forward shock model. Both X-ray and optical emission is consistent with being in the same spectral regime above the cooling frequency ( $ν_{c}$ ). The fact that $ν_{c}$ is below the optical band from the very early epoch of the observation provides a constraint on the burst environment, which is similar to that seen in classical long duration GRBs. We therefore suggest that death of a massive star is the possible progenitor of this short burst.

7 pages, 1 figures, 2 tables, revised version, MNRAS, in press

Probing the Nature of High-z Short GRB 090426 with Its Early Optical and X-ray Afterglows

Abstract

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