The origin of low-redshift event rate excess as revealed by the low-luminosity GRBs

TL;DR

This study investigates the excess of low-redshift long Gamma-Ray Burst rates compared to star formation rates, finding that low-luminosity bursts contribute to this excess, suggesting different origins from star formation.

Contribution

It demonstrates that low-luminosity GRBs at low redshift have an excess rate unrelated to star formation, independent of sample size and completeness.

Findings

Low-luminosity GRB rates exceed star formation rates at z < 1.

High-luminosity GRB rates are consistent with star formation.

Low-redshift excess is likely due to low-luminosity bursts with different origins.

Abstract

The relation between the event rate of long Gamma-Ray Bursts at low redshift and the star formation rate is still controversial, especially in the low-redshift end. Dong et al. confirmed that the Gamma-Ray Burst rate always exceeds the star formation rate at low-redshift of z < 1 in despite of the sample completeness. However, the reason of low-redshift excess is still unclear. Considering low-luminosity bursts with smaller redshift generally, we choose three Swift long burst samples and classify them into low- and high-luminosity bursts in order to check whether the low-redshift excess is existent and if the excess is biased by the sample size and completeness. To degenerate the redshift evolution from luminosity, we adopt the non-parametric method to study the event rate of the two types of long bursts in each sample. It is found that the high-luminosity burst rates are consistent with the star formation rate within the whole redshift range while the event rates of low-luminosity bursts exceed the star formation rate at low redshift of z < 1. Consequently, we conclude that the low-redshift excess is contributed by the low-luminosity bursts with possibly new origins unconnected with the star formation, which is also independent of the sample size and the sample completeness.