A Unified Picture of Short and Long Gamma-Ray Bursts from Compact Binary Mergers

The recent detections of the ∼10 s long γ-ray bursts (GRBs) 211211A and 230307A followed by softer temporally extended emission (EE) and kilonovae point to a new GRB class. Using state-of-the-art first-principles simulations, we introduce a unifying theoretical framework that connects binary neutron star (BNS) and black hole–NS (BH–NS) merger populations with the fundamental physics governing compact binary GRBs (cbGRBs). For binaries with large total masses, M_tot ≳ 2.8 M_⊙, the compact remnant created by the merger promptly collapses into a BH surrounded by an accretion disk. The duration of the pre-magnetically arrested disk (MAD) phase sets the duration of the roughly constant power cbGRB and could be influenced by the disk mass, M_d. We show that massive disks (M_d ≳ 0.1 M_⊙), which form for large binary mass ratios q ≳ 1.2 in BNS or q ≲ 3 in BH–NS mergers, inevitably produce 211211A-like long cbGRBs. Once the disk becomes MAD, the jet power drops with the mass accretion rate as M ̇ ∼ t⁻², establishing the EE decay. Two scenarios are plausible for short cbGRBs. They can be powered by BHs with less massive disks, which form for other q values. Alternatively, for binaries with M_tot ≲ 2.8 M_⊙, mergers should go through a hypermassive NS (HMNS) phase, as inferred for GW170817. Magnetized outflows from such HMNSs, which typically live for ≲1 s, offer an alternative progenitor for short cbGRBs. The first scenario is challenged by the bimodal GRB duration distribution and the fact that the Galactic BNS population peaks at sufficiently low masses that most mergers should go through an HMNS phase.

We thank the referee, Alexander Tchekhovskoy, Rosalba Perna, Jonatan Jacquemin-Ide, Om Sharan Salafia, and Daniel Kasen for valuable discussions. We thank Jillian Rastinejad for providing the observational data for GRB 211211A. O.G. is supported by Flatiron Research and CIERA Fellowships. O.G. acknowledges support by Fermi Cycle 14 Guest Investigator program 80NSSC22K0031 and NSF grant AST-2107839. B.D.M. acknowledges support from the National Science Foundation (grant No. AST-2002577). D.I. is supported by Future Investigators in NASA Earth and Space Science and Technology (FINESST) award No. 80NSSC21K1851. An award of computer time was provided by the ASCR Leadership Computing Challenge (ALCC), Innovative and Novel Computational Impact on Theory and Experiment (INCITE), and OLCF Director’s Discretionary Allocation programs under award PHY129. This research used resources of the National Energy Research Scientific Computing Center, a DOE Office of Science User Facility supported by the Office of Science of the U.S. Department of Energy under contract No. DE-AC02-05CH11231 using NERSC awards ALCC-ERCAP0022634 and NP-ERCAP0020543 (allocation m2401). This research was facilitated by the Multimessenger Plasma Physics Center (MPPC), NSF grant PHY-2206610.