GRB 090423: Marking the Death of a Massive Star at z=8.2
  GRB 090423 is the new high-z record holder of Gamma-ray bursts (GRBs) with z~
8.2. We present a detailed analysis of both the spectral and temporal features
of GRB 090423 observed with Swift/BAT and Fermi/GBM. We find that the T90
observed with BAT in the 15-150 keV band is 13.2 s, corresponding to ~ 1.4 s at
z=8.2. It once again gives rise to an issue whether the progenitors of high-z
GRBs are massive stars or mergers since the discovery of GRB 080913 at z=6.7.
In comparison with T90 distribution in the burst frame of current
redshift-known GRB sample, we find that it is marginally grouped into the long
group (Type II GRBs). The spectrum observed with both BAT and GBM is well
fitted by a power-law with exponential cutoff, which yields an Ep=50.4+/-7.0
keV. The event well satisfies the Amati-relation for the Type II GRBs within
their 3 siggma uncertainty range. Our results indicate that this event would be
produced by the death of a massive star. Based on the Amati-relation, we derive
its distance modulus, which follows the Hubble diagram of the concordance
cosmology model at a redshift of ~8.2.
