The Luminosity Function of Long Gamma-Ray Bursts and their rate at z>6

TL;DR

This paper investigates the luminosity function and formation rate of long gamma-ray bursts under different models, finding that luminosity evolution with redshift is necessary to explain observed data and proposing a method to use high-redshift GRB afterglows to study reionization.

Contribution

It compares three models of GRB evolution and demonstrates that luminosity evolution is essential, also proposing a novel approach to constrain reionization using GRB afterglow spectra.

Findings

Scenario i) is ruled out by Swift data.

Luminosity evolution with redshift is supported by the data.

Method proposed to use high-z GRB afterglows to study reionization.

Abstract

We compute the luminosity function (LF) and the formation rate of long gamma ray bursts (GRBs) in three different scenarios: i) GRBs follow the cosmic star formation and their LF is constant in time; ii) GRBs follow the cosmic star formation but the LF varies with redshift; iii) GRBs form preferentially in low-metallicity environments. We then test model predictions against the Swift 3-year data, showing that scenario i) is robustly ruled out. Moreover, we show that the number of bright GRBs detected by Swift suggests that GRBs should have experienced some sort of luminosity evolution with redshift, being more luminous in the past. Finally we propose to use the observations of the afterglow spectrum of GRBs at z>5.5 to constrain the reionization history and we applied our method to the case of GRB 050904.