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

¹ Department of Astronomy, School of Physics, Peking University, Beijing 100871, China
² Kavli Institute for Astronomy and Astrophysics, Peking University, Beijing 100871, China
³ Department of Physics, University of Rome “Tor Vergata”, Via della Ricerca Scientifica 1, 00100, Rome, Italy
⁴ School of Physics and Astronomy, Monash University, Clayton, VIC 3800, Australia
⁵ OzGrav: The ARC Centre of Excellence for Gravitational Wave Discovery, Australia
⁶ Nevada Center for Astrophysics, University of Nevada, Las Vegas, NV 89154, USA
⁷ Department of Physics and Astronomy, University of Nevada, Las Vegas, NV 89154, USA
⁸ National Astronomical Observatories, Chinese Academy of Sciences, Beijing 100012, China

^⋆ Corresponding authors: yckang@stu.pku.edu.cn; lshao@pku.edu.cn

Received: 10 March 2025
Accepted: 15 May 2025

Abstract

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.