2011MNRAS.410...27X

GRB 090426 is a short-duration burst detected by Swift(T₉₀ ∼ 1.28 s in the observer frame and T₉₀ ∼ 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 gamma-ray bursts (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 Tsinghua University-National Astronomical Observatory of China Telescope (TNT) at Xinglong Observatory and the 1-m LOAO telescope at Mt Lemmon Optical Astronomy Observatory in Arizona. Our well-sampled optical afterglow light curve covers from ∼ 90 to 10⁴ s after the GRB trigger. It shows two shallow decay episodes that are likely due to energy injection, which end at ∼ 230 and 7100 s, respectively. The decay slopes after the injection phases are consistent with each other (α~= 1.22). The X-ray afterglow light curve appears to trace the optical, although the second energy-injection phase was missed due to visibility constraints introduced by the Swift orbit. The X-ray spectral index is β_x∼1.0 without temporal evolution. Its decay slope is consistent with the prediction of the forward shock model. Both X-ray and optical emission are 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.