Short Spike, Long Story: Episode-Dependent Shifts of Long-Duration Type-I GRBs on Ep,z--Eiso Plane
Abstract
Observations of peculiar GRBs have challenged the traditional T90-based classification, demonstrating that duration does not map uniquely onto progenitor type. A striking class is long-duration Type~I GRBs -- merger-origin events whose prompt emission lasts far longer than the canonical 2 s boundary, typically comprising an initial short hard spike followed by softer extended emission. Identifying the physical origin of such bursts requires diagnostics beyond duration alone, among which the Amati relation, linking rest-frame spectral peak energy Ep,z and isotropic-equivalent energy Eiso, is widely used as a complementary classification tool. We analyze a sample of eight long-duration Type~I GRBs and merger candidates by separating the initial spike from the extended emission and examining their episode-dependent locations on the Ep,z--Eiso plane. We find that the initial spike generally lies within, or close to, the empirical Type~I region, consistent with a compact-merger-like prompt-emission component. In contrast, the extended-emission episode systematically occupies a region closer to Type~II GRBs, and could be misidentified as collapsar-like if analyzed in isolation. This episode-dependent Type~I-to-Type~II transition is further supported by time-resolved spectral analysis, although its magnitude and trajectory vary among bursts, suggesting diversity in central-engine evolution or outflow properties between the two phases. Our results caution that the Amati relation alone can lead to misleading empirical classification when the initial hard spike is weak, outside the instrumental bandpass, or missed entirely, leaving only the extended emission to be analyzed. Broad temporal and spectral coverage, and independent multi-wavelength diagnostics, is therefore essential for identifying the physical origin of these events.
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BibTeX
@misc{jiang2026short,
title = {{Short Spike, Long Story: Episode-Dependent Shifts of Long-Duration Type-I GRBs on $E_\rm p,z$--$E_\rm iso$ Plane}},
author = {Ya-Hui Jiang and Run-Chao Chen and Zhen-Yu Yan and Kamil Nadaf and Xiao-Hong Zhao and Bin-Bin Zhang},
howpublished = {arXiv},
year = {2026},
month = sep,
doi = {10.48550/arxiv.2606.15098},
eprint = {2606.15098},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2606.15098},
}
RIS
TY - UNPB
TI - Short Spike, Long Story: Episode-Dependent Shifts of Long-Duration Type-I GRBs on E_p,z--E_iso Plane
AU - Ya-Hui Jiang
AU - Run-Chao Chen
AU - Zhen-Yu Yan
AU - Kamil Nadaf
AU - Xiao-Hong Zhao
AU - Bin-Bin Zhang
PY - 2026
DA - 2026/09/23
PB - arXiv
DO - 10.48550/arxiv.2606.15098
UR - https://arxiv.org/abs/2606.15098
AB - Observations of peculiar GRBs have challenged the traditional T_90-based classification, demonstrating that duration does not map uniquely onto progenitor type. A striking class is long-duration Type~I GRBs -- merger-origin events whose prompt emission lasts far longer than the canonical 2 s boundary, typically comprising an initial short hard spike followed by softer extended emission. Identifying the physical origin of such bursts requires diagnostics beyond duration alone, among which the Amati relation, linking rest-frame spectral peak energy E_p,z and isotropic-equivalent energy E_iso, is widely used as a complementary classification tool. We analyze a sample of eight long-duration Type~I GRBs and merger candidates by separating the initial spike from the extended emission and examining their episode-dependent locations on the E_p,z--E_iso plane. We find that the initial spike generally lies within, or close to, the empirical Type~I region, consistent with a compact-merger-like prompt-emission component. In contrast, the extended-emission episode systematically occupies a region closer to Type~II GRBs, and could be misidentified as collapsar-like if analyzed in isolation. This episode-dependent Type~I-to-Type~II transition is further supported by time-resolved spectral analysis, although its magnitude and trajectory vary among bursts, suggesting diversity in central-engine evolution or outflow properties between the two phases. Our results caution that the Amati relation alone can lead to misleading empirical classification when the initial hard spike is weak, outside the instrumental bandpass, or missed entirely, leaving only the extended emission to be analyzed. Broad temporal and spectral coverage, and independent multi-wavelength diagnostics, is therefore essential for identifying the physical origin of these events.
ER -