2007ApJ...671.1896A

Several luminosity indicators have been found for gamma-ray bursts (GRBs) wherein measurable light-curve and spectral properties are well correlated with the peak luminosity. Several papers have each applied one different luminosity relation to find redshifts for BATSE GRBs and claim to identify specific bursts with z>8. The existence of such high-redshift events is not surprising, as BATSE has enough sensitivity to see them and GRBs are expected out to the redshift of the first star formation. To improve results we used five luminosity relations with updated calibrations to determine redshifts with error bars. Combining these relations, we calculated the redshifts of 36 BATSE GRBs with claimed z>8. Our results include 13 bursts with our derived best redshift z_best>8, which looks promising at first. But the calculated redshift uncertainties are significantly large in these selected cases. With only one exception, all of our bursts have z_1σlow≤9. The one exception (BATSE trigger 2035) is likely a short-duration burst at z≳4. Our best case for a very high redshift event is BATSE trigger 3142 with z_best≥20 and z_1σlow=8.9; however, we can only say z>4.1 at the 2 σ confidence level. In all, we cannot point toward any one BATSE burst as confidently having z>8. One implication is to greatly weaken prior claims that GRBs have a steeply rising rate density out to high redshifts.