Gamma-ray bursts divide into two classes: short bursts from compact object mergers and long bursts from massive star collapses. The classification — duration above or below about two seconds — has served as a reliable proxy for the physical origin. GRB 230307A complicated the picture: a long-duration burst that showed all the hallmarks of a merger origin, including a kilonova afterglow.
Maccary and collaborators ask: how many more are hiding? They analyze the temporal structure of long GRBs in the Fermi/GBM catalogue, looking for the specific pattern of light-curve peaks, spectral evolution, and minimum variability timescales that characterize GRB 230307A. Nine candidates emerge — long-duration bursts whose temporal fingerprints match the merger template.
The candidates fail the standard collapsar relations (Amati, Yonetoku) that long GRBs typically satisfy, and their minimum variability timescales cluster with known merger events rather than with the collapsar population. The temporal properties serve as a diagnostic that the simple duration cut misses.
The fraction of misclassified mergers among long GRBs may be larger than previously assumed. A hidden population has been sitting in the catalogues, sorted into the wrong bin by a classification scheme that treated duration as sufficient. Duration is a projection of the underlying physics, and as with any projection, it loses information. The temporal microstructure — the pattern of peaks, their spacing, their spectral evolution — encodes the origin more faithfully than the total length. The classification was measuring the shadow, not the object.