A Unified Picture of Short and Long Gamma-ray Bursts from Compact Binary Mergers, Gottlieb et al. (2023)
The recent detections of the ~10 s long gamma-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 Mtot>2.8 Msun, the compact remnant created by the merger promptly collapses into a BH, surrounded by an accretion disk. The duration of the magnetically arrested disk (MAD) phase sets the duration of the roughly constant power cbGRB and could be influenced by the disk mass, Md: long cbGRBs such as 211211A are produced by massive disks Md>0.1 Msun, which form for large binary mass ratio q>1.2 in BNS or q<3 in BH-NS mergers. Once the disk becomes MAD, the jet power drops with the mass accretion rate as Mdot~1/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 Mtot<2.8 Msun, mergers should go through a hypermassive NS (HMNS) phase, as inferred for GW170817. Magnetized outflows from such HMNSs, which typically live for < 1 second, offer an alternative progenitor for short cbGRBs. The first scenario is challenged by the bimodal distribution of GRB durations and the fact that the Galactic BNS population peaks at sufficiently low masses that most mergers should go through a HMNS phase. HMNS-powered jets also more readily account for other light curve features, from precursor flares to EE characteristics.
A Unified Model of Kilonovae and GRBs in Binary Mergers Establishes Neutron Stars as the Central Engines of Short GRBs, Gottlieb et al. (2024)
We expand the theoretical framework by Gottlieb et al. (2023), which connects binary merger populations with long and short binary gamma-ray bursts (lbGRBs and sbGRBs), incorporating kilonovae as a key diagnostic tool. We show that lbGRBs, powered by massive accretion disks around black holes (BHs), should be accompanied by bright, red kilonovae. In contrast, sbGRBs – if also powered by BHs – would produce fainter, red kilonovae, potentially biasing against their detection. However, magnetized hypermassive neutron star (HMNS) remnants that precede BH formation can produce jets with power and Lorentz factor compatible with sbGRB observations, and would result in distinctly bluer kilonovae, offering a pathway to identifying the sbGRB central engine. Recent modeling by Rastinejad et al. (2024) found luminous red kilonovae consistently accompany lbGRBs, supporting lbGRB originating from BH-massive disk systems, likely following a short-lived HMNS phase. The preferential association of sbGRBs with comparably luminous kilonovae argues against the BH engine hypothesis for sbGRBs, while the bluer hue of these KNe provides additional support for an HMNS-driven mechanism. Within this framework, BH–NS mergers likely contribute exclusively to the lbGRB population with red kilonovae. Our findings suggest that GW170817 may, in fact, have been an lbGRB to on-axis observers. Finally, we discuss major challenges faced by alternative lbGRB progenitor models, such as white dwarf–NS or white dwarf–BH mergers and accretion-induced collapse forming magnetars, which fail to align with observed GRB timescales, energies, and kilonova properties.