TITLE: GCN CIRCULAR NUMBER: 9279 SUBJECT: GRB 090423: pseudo burst at z=1 and its relation to GRB 080913 DATE: 09/04/29 00:10:01 GMT FROM: Binbin Zhang at UNLV Bin-Bin Zhang and Bing Zhang (University of Nevada Las Vegas) report: GRB 090423 is a second, high-z, intrinsically short GRB after GRB 080913. Following the similar procedure discussed in Zhang, B et al 2009, http://arxiv.org/abs/0902.2419 , we have simulated a pseudo GRB by shifting GRB 090423 to z=1. The following three factors, i.e, specific flux (f_\nu) amplification due to a smaller luminosity distance, blue-shift of spectrum, and temporal compression of the lightcurve (de-dilation), have been considered. We notice that there is an early X-ray flare in GRB 090423 (again similar to GRB 080913), which should be harder and observable by BAT if it were at z=1. Following the similar procedure described in Zhang et al. (2009), we manipulate the XRT data of GRB 090423 to simulate the BAT band extended emission of the pseudo burst. The constructed BAT band lightcurve of the pseudo GRB at z=1 is shown in the figure at http://grb.physics.unlv.edu/gcns/090423/pseudo.jpg . This psuedo burst appears as a short duration GRB with extended emission. On the other hand, both high-z GRBs have high isotropic luminosity and energy, which make them following the Amati/Yonetoku correlation defined by GRBs that are of the massive star origin (Type II or long population). Although it is possible to have NS-NS and NS-BH mergers (Type I or short population) at such a high-z (Belczynski et al. 2009), the difficulty is to have two such energetic merger events at high-z. The Type I model has difficulty to accommodate both low-z, low-L Type I events and these events in terms of luminosity function (Zhang et al. 2009). Based on the multiple criteria analysis, we cannot address the physical category of GRB 090423 and GRB 080913 using more definite criteria (e.g. SN association, host galaxy property, etc). On the other hand, one can use less definite criteria (Amati/Yonetoku relation and energetics) to judge that both bursts are Type II (massive star collapse) candidates. A judging flow chart using Fig. 8 of Zhang et al. (2009) is posted at http://grb.physics.unlv.edu/gcns/090423/flowchart.jpeg . We also noticed the interesting discussions by Krimm et al. (GCN 9241) and Nava et al. (GCN 9235), which are broadly consistent with our conclusion here. Finally, using the three samples (Type II Gold, Type I Gold, and Other Short/Hard) defined in Zhang et al. (2009), we plot the intrinsic duration (T_{90}/(1+z)) as a function of z (see http://grb.physics.unlv.edu/gcns/090423/t90z.png ). It is interesting to note that the two high-z bursts are intrinsically shorter than the majority of Type II GRBs. If these observations persist in the future, it may suggest an intrinsic trend of short duration for Type II GRBs at high-z. This message can be cited.