Probing the nature of high-z short GRB 090426 with its early optical and X-ray afterglows

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 $\gamma$-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 $\sim 10^4$ 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 ($\alpha\simeq 1.22$). 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 $\beta_X\sim 1.0$ 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 ($\nu_c$). The fact that $\nu_c$ 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.