Cosmological Evolution of Long Gamma-Ray Bursts and the Star Formation Rate

Gamma-ray bursts (GRBs), by virtue of their high luminosities, can be detected up to very high redshifts and therefore can be excellent probes of the early universe. This task is hampered by the fact that most of their characteristics have a broad range, so we first need to obtain an accurate description of the distribution of these characteristics and, especially, their cosmological evolution. We use a sample of about 200 Swift long GRBs with known redshifts to determine the evolution of the luminosity, formation rate, and the general shape of the luminosity function (LF). In contrast to most other forward-fitting methods of treating this problem, we use the Efron–Petrosian methods, which allow a non-parametric determination of the above quantities. We find a relatively strong luminosity evolution, an LF that can be fitted to a broken power law, and an unusually high formation rate at low redshifts, a rate more than one order of magnitude higher than the star formation rate (SFR). On the other hand, our results seem to agree with the almost constant SFR in redshifts 1–3 and the decline above this redshift.