Shared properties of merger-driven long-duration gamma-ray bursts

Context. The recent detections of bright optical/infrared kilonova signals following two long-duration gamma-ray bursts (LGRBs), GRB 211211A and GRB 230307A, have significantly challenged the traditional classification of GRBs. These merger-driven LGRBs may represent a distinct GRB population, sparking interest in their progenitors and central engines. Aims. Traditional GRB classification methods often struggle to distinguish merger-driven LGRBs from traditional merger-driven short-duration GRBs resulting from compact object mergers and from collapse-driven LGRBs produced by massive stars. We thus aim to explore the shared properties in terms of hardness, energy, and duration among observed merger-driven LGRB events, thereby identifying their observed differences from the traditional GRB population. Methods. We collected a sample of merger-driven LGRBs with known redshifts, including observed information on their main emission (ME) and whole emission (WE) phases. Treating ME and WE properties as two independent sets of information, we applied several GRB classification methodologies to explore their potential shared properties. Results. Using the phenomenologically defined energy-hardness ( EH ) parameter, characterized by the intrinsic hardness and energy of GRBs, and the duration of GRBs, we identified a probable universal linear correlation across merger-driven LGRBs that holds regardless of whether their ME or WE phases are considered. Conclusions. We propose that such shared properties of merger-driven LGRBs are unlikely to arise from the low-redshift selection effect, and they become particularly intriguing when compared with the relatively weak correlations or lack of correlation observed in traditional merger-driven short-duration GRBs (with or without extended emissions) and collapse-driven LGRBs. Our newly proposed correlation highlights the necessity for further investigation into the observations of merger-driven LGRBs and the physical mechanisms underlying the empirical correlation.