Spectral Hardness as the Primary Discriminator: Unveiling the Collapsar–Merger Boundary with a Gold-standard Gamma-Ray Burst Sample

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Abstract Gamma-ray bursts (GRBs) are promising cosmological probes. However, their utility is hampered by sample impurity arising from traditional duration-based classification. The existence of “hybrid” events—such as the merger-origin GRBs 060614 and 211211A that exhibit extended durations ( T 90 ≫ 2 s)—highlights the limitations of the T 90 parameter as a sole discriminator. In this Letter, we establish a robust, physically motivated classification method using a support vector machine (SVM) trained on a “gold-standard” sample of 24 GRBs with spectroscopically confirmed progenitors (associated supernovae or kilonovae). By isolating the prompt main spike to excise contamination from extended emission, we derive a quantitative classification index, I SV M = 5.01 log 10 E p , i − 1.25 log 10 E iso − 0.34 log 10 T 90 , z − 12.90 (units: keV, 10 52 erg s). Events with I SVM > 0 are classified as mergers. Analysis of the standardized classification weights reveals that the discriminative power of E p , i is approximately 5 times that of T 90, z , while E iso contributes a weight comparable to E p , i . This quantitatively demonstrates that spectral hardness and energetics, rather than duration, are the primary physical signatures distinguishing mergers from collapsars. The derived boundary implies a stringent hardness ceiling for collapsars, while mergers are identified as outliers with excessive hardness relative to their energy budget. The classifier successfully identifies the nature of historic test cases, including the ultralong GRB 111209A (collapsar) and the short GRB 050709 (merger), independent of instrumental eras. This tool paves the way for cleaning archival and future high-redshift GRB samples for precision cosmology.

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