Instrumental Tip-of-the-iceberg Effects on the Prompt Emission of Swift/BAT Gamma-ray Bursts

TL;DR

This study quantifies how instrumental effects and observing conditions, especially source incident angle, bias the measured durations of gamma-ray burst prompt emissions observed by Swift/BAT, often underestimating true durations.

Contribution

It introduces a detailed simulation framework to assess how Swift/BAT's instrumental and observational factors influence GRB duration measurements, highlighting the significant bias introduced by the tip-of-the-iceberg effect.

Findings

Duration measurements are highly sensitive to observing conditions.

Incident angle has the highest impact on measurement bias.

Most synthetic durations are significantly shorter than true durations.

Abstract

The observed durations of prompt gamma-ray emission from gamma-ray bursts (GRBs) are often used to infer the progenitors and energetics of the sources. Inaccurate duration measurements will have a significant impact on constraining the processes powering the bursts. The "tip-of-the-iceberg" effect describes how the observed signal is lost into background noise; lower instrument sensitivity leads to higher measurement bias. In this study, we investigate how observing conditions, such as the number of enabled detectors, background level, and incident angle of the source relative to the detector plane, affect the measured duration of GRB prompt emission observed with the Burst Alert Telescope on board the Neil Gehrels Swift Observatory (Swift/BAT). We generate "simple-pulse" light curves from an analytical fast rise exponential decay function and from a sample of eight real GRB light curves. We fold these through the Swift/BAT instrument response function to simulate light curves Swift/BAT would have observed for specific observing conditions. We find duration measurements are highly sensitive to observing conditions and the incident angle of the source has the highest impact on measurement bias. In most cases duration measurements of synthetic light curves are significantly shorter than the true burst duration. For the majority of our sample, the percentage of duration measurements consistent with the true duration is as low as ~25%-45%. In this article, we provide quantification of the tip-of-the-iceberg effect on GRB light curves due to Swift/BAT instrumental effects for several unique light curves.