GRB 090423: Marking the Death of a Massive Star at z=8.2

TL;DR

GRB 090423 at redshift 8.2 provides evidence that high-redshift gamma-ray bursts originate from massive star deaths, with spectral and temporal analysis supporting their classification as Type II GRBs and consistent with cosmological models.

Contribution

This study presents detailed spectral and temporal analysis of the highest redshift GRB, confirming its origin from a massive star and supporting the use of GRBs as cosmological probes.

Findings

GRB 090423's T90 is 13.2 s in 15-150 keV band, corresponding to ~1.4 s at z=8.2.

Spectrum fits a power-law with exponential cutoff, Ep=50.4 keV.

GRB 090423 satisfies the Amati relation for Type II GRBs.

Abstract

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.