The luminosity function and the rate of Swift's Gamma Ray Bursts

TL;DR

This paper derives the luminosity function and rate evolution of long Swift gamma-ray bursts by directly analyzing their redshift and luminosity distribution, revealing a broken power law behavior and consistency with high-redshift observations.

Contribution

It provides a novel direct inversion method to determine GRB rate and luminosity function, extending predictions to high redshifts and comparing with star formation rates.

Findings

GRB rate rises as (1+z)^2.1 for 0<z<3

Luminosity function is a broken power law with a break at 10^52.5 erg/sec

GRB 090423 at z~8 fits the model's high-redshift prediction

Abstract

We invert directly the redshift - luminosity distribution of observed long Swift GRBs to obtain their rate and luminosity function. Our best fit rate is described by a broken power law that rises like (1+z)^2.1{+0.5-0.6} for 0<z<3 and decrease like (1+z)^-1.4{+2.4-1.0} for z>3. The local rate is 1.3^{+0.6-0.7} [Gpc^-3 yr^-1]. The luminosity function is well described by a broken power law with a break at L* = 10^52.5{+-0.2}[erg/sec] and with indices alpha = 0.2^{+0.2-0.1} and beta = 1.4^{+0.3-0.6}. The recently detected GRB 090423, with redshift ~8, fits nicely into the model's prediction, verifying that we are allowed to extend our results to high redshifts. While there is a possible agreement with the star formation rate (SFR) for z<3, the high redshift slope is shallower than the steep decline in the SFR for 4<z. However we cannot rule out a GRB rate that follows one of the recent SFR models.