Short Spike, Long Story: Episode-dependent Shifts of Long-duration Type-I Gamma-Ray Bursts on the E p,z – E iso Plane

Abstract Observations of gamma-ray bursts (GRBs) have increasingly challenged the traditional T 90 -based classification, demonstrating that duration does not map uniquely onto progenitor type. A particularly striking class consists of long-duration Type I GRBs (merger-origin) events whose prompt emission lasts far longer than the 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 E p,z – E iso relation 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 subsequent 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 that of Type II GRBs, and could be misidentified as collapsar-like if analyzed in isolation. This Type I-to-Type II transition is further supported by time-resolved spectral analysis, suggesting diversity in central-engine evolution or outflow properties between the two emission phases. Our results caution that the Amati relation alone can lead to a misleading empirical classification when the initial hard spike is weak, falls outside the instrumental bandpass, or is missed entirely, leaving only the extended emission to be analyzed. Broad temporal and spectral coverage, together with independent multiwavelength diagnostics, is therefore essential for identifying the physical origin of these complex events.