Probing the Gamma-Ray Burst Rate with Trigger Simulations of the Swift Burst Alert Telescope

TL;DR

This paper develops a detailed simulation of the Swift GRB trigger algorithm to better estimate the intrinsic gamma-ray burst rate, revealing potential higher rates or luminosity evolution at high redshift.

Contribution

It introduces a comprehensive simulation tool for Swift's complex trigger criteria, improving estimates of the intrinsic GRB rate and its implications for cosmic star formation.

Findings

Adopting the complex trigger algorithm increases detection of dim bursts.

The intrinsic GRB rate may be higher at large redshift than previously thought.

Luminosity evolution could be significant in understanding GRB observations.

Abstract

The long gamma-ray burst (GRB) rate is essential for revealing the connection between GRBs, supernovae and stellar evolution. Additionally, the GRB rate at high redshift provides a strong probe of star formation history in the early universe. While hundreds of GRBs are observed by Swift, it remains difficult to determine the intrinsic GRB rate due to the complex trigger algorithm of Swift. Current studies usually approximate the Swift trigger algorithm by a single detection threshold. However, unlike the previously flown GRB instruments, Swift has over 500 trigger criteria based on photon count rate and additional image threshold for localization. To investigate possible systematic biases and explore the intrinsic GRB properties, we developed a program that is capable of simulating all the rate trigger criteria and mimicking the image trigger threshold. We use this program to search for the intrinsic GRB rate. Our simulations show that adopting the complex trigger algorithm of Swift increases the detection rate of dim bursts. As a result, we find that either the GRB rate is much higher than previously expected at large redshift, or the luminosity evolution is non-negligible. We will discuss the best results of the GRB rate in our search, and their impact on the star-formation history.