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

TL;DR

This study analyzes the pulse-by-pulse properties of long-duration GRBs observed by Swift, revealing differences between precursors and main bursts that suggest distinct dissipation conditions but a common collapsar origin.

Contribution

It provides the first systematic pulse-resolved comparison of precursors and main bursts in long GRBs, highlighting their differing temporal and spectral characteristics.

Findings

Precursors have longer minimum variability timescales than main bursts.

Precursors exhibit near-zero spectral lags, unlike the positive lags of main bursts.

Both components are consistent with a single collapsar progenitor.

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.