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Pulse-resolved Classification and Characteristics of Long-duration GRBs with \emph{Swift}-BAT Data.I. Precursors versus Main Bursts

Liang Li, Yu Wang, Jin-Jun Geng, Yong-Feng Huang, Rong-Gen Cai

TL;DR

This work addresses the challenge of classifying complex, multi-episode long GRBs by performing pulse-resolved analysis on 22 Swift-BAT GRBs with precursors. The authors extract episode-level observables—$T_{90}$, hardness, minimum variability timescale (MVT), and spectral lag—for each precursor ($G_1$) and main burst ($G_2$), and compare them against Type I/II reference distributions. They find that both episodes obey Type II characteristics, but precursors have systematically longer MVT and more diverse, often near-zero lags, while main bursts show shorter variability and positive lags, implying different dissipation scales within a single collapsar jet and possible cocoon shock breakout for the precursor. The results support a two-phase collapsar picture and demonstrate the value of pulse-resolved classification for probing jet formation and pre-burst activity, with implications for extending this approach to other complex GRBs and broader energy ranges.

Abstract

We present a systematic pulse-by-pulse analysis of 22 long-duration GRBs observed by \emph{Swift}, each exhibiting a well-separated precursor before the main burst. We compare duration, spectral hardness ratio, minimum variability timescale (MVT), and spectral lag between these components. Both precursors and main bursts have durations and hardness broadly consistent with Type II GRBs. However, precursors show longer MVTs (by factors of 3-10) and diverse lags with near-zero median values, while main bursts display variable MVTs and positive lags. These differences suggest precursors may originate from distinct dissipation conditions, possibly due to cocoon shock breakout or early magnetically dominated outflows. Despite temporal differences, both episodes are consistent with a single collapsar origin, providing no evidence for dual-progenitor events. Our findings support pulse-resolved classification and show that precursors offer critical insights into jet formation and pre-burst activity.

Pulse-resolved Classification and Characteristics of Long-duration GRBs with \emph{Swift}-BAT Data.I. Precursors versus Main Bursts

TL;DR

This work addresses the challenge of classifying complex, multi-episode long GRBs by performing pulse-resolved analysis on 22 Swift-BAT GRBs with precursors. The authors extract episode-level observables—, hardness, minimum variability timescale (MVT), and spectral lag—for each precursor () and main burst (), and compare them against Type I/II reference distributions. They find that both episodes obey Type II characteristics, but precursors have systematically longer MVT and more diverse, often near-zero lags, while main bursts show shorter variability and positive lags, implying different dissipation scales within a single collapsar jet and possible cocoon shock breakout for the precursor. The results support a two-phase collapsar picture and demonstrate the value of pulse-resolved classification for probing jet formation and pre-burst activity, with implications for extending this approach to other complex GRBs and broader energy ranges.

Abstract

We present a systematic pulse-by-pulse analysis of 22 long-duration GRBs observed by \emph{Swift}, each exhibiting a well-separated precursor before the main burst. We compare duration, spectral hardness ratio, minimum variability timescale (MVT), and spectral lag between these components. Both precursors and main bursts have durations and hardness broadly consistent with Type II GRBs. However, precursors show longer MVTs (by factors of 3-10) and diverse lags with near-zero median values, while main bursts display variable MVTs and positive lags. These differences suggest precursors may originate from distinct dissipation conditions, possibly due to cocoon shock breakout or early magnetically dominated outflows. Despite temporal differences, both episodes are consistent with a single collapsar origin, providing no evidence for dual-progenitor events. Our findings support pulse-resolved classification and show that precursors offer critical insights into jet formation and pre-burst activity.
Paper Structure (14 sections, 9 equations, 2 tables)

This paper contains 14 sections, 9 equations, 2 tables.

Table of Contents

  1. Introduction
  2. Methodology
  3. Sample Selection
  4. Bivariate Normal Distribution
  5. Temporal Properties of Sub-Bursts
  6. Results
  7. Duration and Temporal Structure
  8. Hardness Ratio
  9. Minimum Variability Timescale
  10. Spectral Lag
  11. Discussion
  12. Minimum variability timescale as a radius diagnostic for $G_1$ vs. $G_2$
  13. Spectral lag: weak spectral evolution in $G_1$ vs. pronounced evolution in $G_2$
  14. Conclusion