Redshift Degeneracy in the Eiso-Epeak Relation of Gamma-Ray Bursts
  In this Letter we show that there is a redshift degeneracy in the Eiso-Epeak relation of gamma-ray bursts (GRBs). If a GRB has a redshift solved from the Eiso-Epeak relation that lies in the range of 0.9<z<20, a GRB that has the same observed fluence and peak spectral energy but is at a different redshift in that range also satisfies the Eiso-Epeak relation within 1-sigma error, implying an extremely large error in the calculated redshift. Even if the data scatter in the Eiso-Epeak relation is reduced by a factor of 2, the error in the predicted redshift is still large enough to prevent constraining the redshift meaningfully. Hence, the Eiso-Epeak relation is not useful for determining the GRB distance. 
Li-Xin Li (MPA)