An Intertwined Short and Long GRB with 4-minute Separation

TL;DR

This paper reports a unique gamma-ray burst with two sub-bursts separated by four minutes, exhibiting both short and long GRB characteristics, challenging existing classification models.

Contribution

It presents the first observed case of a single GRB displaying both merger and collapsar signatures, prompting a re-evaluation of GRB progenitor theories.

Findings

The event shows a short-duration burst followed by a long-duration burst separated by four minutes.

Standard diagnostics classify the two sub-bursts as Type I and Type II GRBs respectively.

The coexistence of merger and collapsar signatures in one event challenges current GRB classification schemes.

Abstract

Gamma-ray bursts (GRBs), the most energetic transients in the Universe, are traditionally classified into long-duration ($T_{90}>2$ s) and short-duration ($T_{90}<2$ s) events, associated with the core collapse of massive stars (Type II) and the merger of compact binary systems (Type I), respectively. The two classes exhibit distinct observational properties that serve as key diagnostic criteria for classification. Here we report GRB 160425A, a peculiar event comprising two sub-bursts separated by four minutes: a short-duration burst ($G_1$) and a long-duration burst ($G_2$). Nearly all standard prompt-emission diagnostics, including pulse morphology, duration, hardness ratio, minimum variability timescale, spectral properties, and established empirical correlations, consistently categorize $G_1$ as a short-like (Type I, merger-origin) and $G_2$ as a long-like (Type II, collapsar-origin) GRB. The coexistence of merger and collapsar signatures in a single event challenges existing progenitor frameworks and calls for a re-evaluation of GRB classification schemes and progenitor scenarios.