# Revisiting the Redshift Distribution of Gamma Ray Bursts in the Swift Era

## TL;DR

This study revisits the redshift distribution of gamma-ray bursts using an expanded Swift sample, employing a broken power law spectrum, and finds an excess of low-redshift GRBs and low high-redshift fractions consistent with star formation history.

## Contribution

It introduces a revised GRB model with a broken power law spectrum and analyzes over 100 Swift GRBs to better fit redshift and jet opening angle distributions.

## Key findings

- GRB mean redshift is ~1.7 in Swift data.
- Jet opening angles average ~11 degrees in Swift.
- Low high-redshift GRB fraction (<10% at z≥4).

## Abstract

Le & Dermer developed a gamma-ray burst (GRB) model to fit the redshift and the jet opening angle distributions measured with pre-Swift and Swift missions and showed that GRBs do not follow the star formation rate. Their fitted results were obtained without the opening angle distribution from Swift with an incomplete Swift sample, and that the calculated jet opening angle distribution was obtained by assuming a flat $\nu F_\nu$ spectrum. In this paper, we revisit the work done by Le & Dermer with an assumed broken power law GRB spectrum. Utilizing more than 100 GRBs in the Swift sample that include both the observed estimated redshifts and jet opening angles, we obtain a GRB burst rate functional form that gives acceptable fits to the pre-Swift and Swift redshift and jet opening angle distributions with an indication that an excess of GRBs exists at low redshift below $z \approx 2$. The mean redshifts and jet opening angles for pre-Swift~(Swift) are $\langle z \rangle \sim 1.3$ ($1.7$) and $\langle \theta_{\rm j} \rangle \sim 7^o$ ($11^o$), respectively. Assuming a GRB rate density (SFR9), similar to the Hopkins & Beacom star formation history and as extended by Li, the fraction of high-redshift GRBs is estimated to be below 10% and 5% at $z \geq 4$ and $z\geq 5$, respectively, and below 10% at $z \leq 1$.