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 T ₉₀ 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 the issue of 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 the T ₉₀ distribution in the burst frame of the 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 E _p=(50.4±7.0) keV. The event satisfies the Amati-relation well for Type II GRBs within their 3σ 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.