Quantifying the Luminosity Evolution in Gamma-Ray Bursts

We estimate the luminosity evolution and formation rate for more than 900 GRBs by using redshift and luminosity data calculated by Band, Norris, & Bonnell via the lag-luminosity correlation. By applying maximum likelihood techniques, we are able to infer the true distribution of the parent GRB population's luminosity function and density distributions in a way that accounts for detector selection effects. We find that after accounting for data truncation, there still exists a significant correlation between the average luminosity and redshift, indicating that distant GRBs are on average more luminous than nearby counterparts. This is consistent with previous studies showing strong source evolution and also recent observations of underluminous nearby GRBs. We find no evidence for beaming angle evolution in the current sample of GRBs with known redshift, suggesting that this increase in luminosity cannot be due to an evolution of the collimation of gamma-ray emission. The resulting luminosity function is well fit with a single power law of index L^'-1.5, which is intermediate between the values predicted by the power-law and Gaussian structured jet models. We also find that the GRB comoving rate density rises steeply, with a broad peak between 1 = z = 2 followed by a steady decline above z > 3. This rate density qualitatively matches the current estimates of the cosmic star formation rate, favoring a short-lived massive star progenitor model or a binary model with a short delay between the formation of the compact object and the eventual merger.