Evidence for Luminosity Evolution of Long Gamma-ray Bursts in Swift Data

We compute the luminosity function (LF) and the formation rate of long gamma ray bursts (GRBs) by fitting the observed differential peak flux distribution obtained by the BATSE satellite in two different scenarios: i) the GRB luminosity evolves with redshift and ii) GRBs form preferentially in low–metallicity environments. In both cases, model predictions are consistent with the Swift number counts and with the number of detections at z > 2.5 and z > 3.5. To discriminate between the two evolutionary scenarios, we compare the model results with the number of luminous bursts (i.e. with isotropic peak luminosity in excess of 10 erg s) detected by Swift in its first three years of mission. Our sample conservatively contains only bursts with good redshift determination and measured peak energy. We find that models in which GRBs are characterised by a constant LF are ruled out, underproducing the number of luminous GRBs. Only for a metallicity threshold for GRB formation as low as 1/10 Z⊙, the model is marginally consistent with the observed number of bright GRBs. Using these new constraints, we derive robust upper limits on the bright–end of the GRB LF, showing that this can not be steeper than ∼ 2.5. In conclusion, we find that available Swift observations (both the number of high-z GRBs and of bright GRBs) point to a scenario in which GRBs were more luminous in the past as we see them today.