What are Luminosity Indicators Telling Us?

We find that there exists a significant correlation between GRB luminosity and redshift in the sample of 220 Gamma‐Ray Burst (GRB) that have redshifts derived from the luminosity‐variability relationship [1]. In particular, we find that the relation between luminosity and redshift can be parameterized as L ∝ (1 + z) 1.4±∼0.5. We discuss the possible reasons behind this correlation, which could result from either energy or jet opening angle evolution with redshift. In addition, we use non‐parametric statistical techniques to independently estimate the distributions of the luminosity and redshift of bursts, accounting for the luminosity evolution (in contrast to previous studies which have assumed that the luminosity function is independent of redshift). Most significantly, we find a co‐moving rate density of GRBs that continues to increase to (1 + z) > 10. From this estimate of the GRB rate density, we then use the population synthesis codes of [2] to estimate the star formation rate at high redshifts, based on different progenitor models of GRBs. We find that no matter what the progenitor or population synthesis model, the star formation rate increases or remains constant to very high redshifts (z > 10).