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Astrophysics, High Energy Astrophysical Phenomena, astro-ph.HE, Astrophysics, Solar and Stellar Astrophysics, astro-ph.SR
Abstract:
The recent detections of the $\sim10$-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 $M_{\rm tot}\gtrsim2.8\,M_\odot$, 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 ($M_d\gtrsim0.1\,M_\odot$), which
form for large binary mass ratio $q\gtrsim1.2$ in BNS or $q\lesssim3$ 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 $\dot{M}\sim t^{-2}$,
naturally 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 $M_{\rm
tot}\lesssim2.8\,M_\odot$, mergers should go through a hypermassive NS (HMNS)
phase, as inferred for GW170817. Magnetized outflows from such HMNSs, which
typically live for $\lesssim1\,{\rm s}$, 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 a HMNS phase.
