The recent detections of the βΌ10-s long Ξ³-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 Mtotββ³2.8Mββ, 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, Mdβ: long cbGRBs such as 211211A are produced by massive disks
(Mdββ³0.1Mββ), which form for large binary mass ratio
qβ³1.2 in BNS or qβ²3 in BH-NS mergers. Once the disk becomes
MAD, the jet power drops with the mass accretion rate as MΛβΌtβ2,
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 Mtotββ²2.8Mββ, mergers
should go through a hypermassive NS (HMNS) phase, as inferred for GW170817.
Magnetized outflows from such HMNSs, which typically live for β²1s, offer an alternative progenitor for short cbGRBs. The first scenario is
challenged by the bimodal distribution of cbGRB 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