GCN Circular 9279

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
Not found
 
 
) and  Nava 
et al. (GCN 9235
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), 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.

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