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 ( s in the observer frame and 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 to 104 s after the GRB trigger. It shows two shallow decay episodes that are likely due to energy injection, which end at and 7100 s, respectively. The decay slopes after the injection phases are consistent with each other (). 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 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 (). The fact that 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.

Open questions in GRB physics

“…Two high-z GRBs 090423 and 080916 have rest-frame durations shorter than 1s, but are likely related to massive stars [31,32,33,34]. An observer-frame short GRB 090426 was found in many aspects similar to long GRBs [35,36,37]. This suggests that certain observation properties (e.g.…”

IDENTIFYING THE LOCATION IN THE HOST GALAXY OF THE SHORT GRB 111117A WITH THECHANDRASUBARCSECOND POSITION

“…Such populations of large-offset short GRBs has already been suggested by Bloom et al (2007) and Troja et al (2008). However, being localized mainly by 27 Although the redshift of GRB 090426 (the t 90 duration measured by BAT is 1.24) has been measured from the absorption spectroscopy (Levesque et al 2010;Thöne et al 2011), there is growing observational evidence that the progenitor of this GRB is akin to that of long GRBs (e.g., Xin et al 2011;Lü et al 2010). …”

Cosmic Evolution of Long Gamma-Ray Burst Luminosity

“…Although the durations of some GRBs, such as GRB050724 and GRB 060614, are larger than 2 s, we do not include them in our sample since they may be from merger of compact stars (Berger et al 2005;Gehrels et al 2006;Zhang et al , 2009). The afterglow and host galaxy observations of the high-z SGRB GRB090426 (T 1.2 s 90 = ) show that it may be from thecollapse of a massive star (Antonelli et al 2009;Xin et al 2011). We thus include this GRB in our sample.…”