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Abstract Building on a general relativistic magnetohydrodynamic simulation of a short gamma-ray burst (sGRB) jet with initial magnetizationσ₀ = 150, propagating through the dynamical ejecta from a binary neutron star merger, we identify regions of energy dissipation driven by magnetic reconnection and collisionless subshocks within different scenarios. We solve the transport equations for photons, electrons, protons, neutrinos, and intermediate particles up to the photosphere, accounting for all relevant radiative processes, including electron and proton acceleration, and investigate the potential impact of magnetic reconnection occurring in different regions along the jet. We find the photon spectra undergo nonthermal modifications below the photosphere, observable in both on-axis and off-axis emission directions, as well as across different scenarios of energy dissipation and subsequent particle acceleration. Interestingly, the spectral index of the photon energy distribution can vary at most by ∼20% across all different dissipation scenarios. Depending on the dissipation mechanism at play, neutrino signatures may accompany the photon signal, pointing to efficient proton acceleration and shedding light on jet physics. Although our findings are based on one jet simulation, they point to a potential universal origin of the nonthermal features of the Band spectrum observed in sGRBs. 

Abstract The observability of afterglows from binary neutron star mergers occurring within active galactic nuclei (AGN) disks is investigated. We perform 3D GRMHD simulations of a postmerger system and follow the jet launched from the compact object. We use semianalytic techniques to study the propagation of the blast wave powered by the jet through an AGN disk-like external environment, extending to distances beyond the disk scale height. The synchrotron emission produced by the jet-driven forward shock is calculated to obtain the afterglow emission. The observability of this emission at different frequencies is assessed by comparing it to the quiescent AGN emission. In the scenarios where the afterglow could temporarily outshine the AGN, we find that detection will be more feasible at higher frequencies (≳10¹⁴Hz) and the electromagnetic counterpart could manifest as a fast variability in the AGN emission, on timescales less than a day. 

Abstract Disk-fed accretion onto neutron stars can power a wide range of astrophysical sources ranging from X-ray binaries, to accretion-powered millisecond pulsars, ultraluminous X-ray sources, and gamma-ray bursts. A crucial parameter controlling the gas–magnetosphere interaction is the strength of the stellar dipole. In addition, coherent X-ray pulsations in many neutron star systems indicate that the star's dipole moment is oblique relative to its rotation axis. Therefore, it is critical to systematically explore the 2D parameter space of the star's magnetic field strength and obliquity, which is what this work does, for the first time, in the framework of 3D general-relativistic magnetohydrodynamics. If the accretion disk carries its own vertical magnetic field, this introduces an additional factor: the relative polarity of the disk and stellar magnetic fields. We find that depending on the strength of the stellar dipole and the star–disk relative polarity, the neutron star's jet power can either increase or decrease with increasing obliquity. For weak dipole strength (equivalently, high accretion rate), the parallel polarity results in a positive correlation between jet power and obliquity, whereas the antiparallel orientation displays the opposite trend. For stronger dipoles, the relative-polarity effect disappears, and jet power always decreases with increasing obliquity. The influence of the relative polarity gradually disappears as obliquity increases. Highly oblique pulsars tend to have an increased magnetospheric radius, a lower mass accretion rate, and enter the propeller regime at lower magnetic moments than aligned stars. 

Abstract The spin of a newly formed black hole (BH) at the center of a massive star evolves from its natal value due to two competing processes: accretion of gas angular momentum that increases the spin and extraction of BH angular momentum by outflows that decreases the spin. Ultimately, the final, equilibrium spin is set by a balance between both processes. In order for the BH to launch relativistic jets and power aγ-ray burst (GRB), the BH magnetic field needs to be dynamically important. Thus, we consider the case of a magnetically arrested disk (MAD) driving the spin evolution of the BH. By applying the semianalytic MAD BH spin evolution model of Lowell et al. to collapsars, we show that if the BH accretes ∼20% of its initial mass, its dimensionless spin inevitably reaches small values,a≲ 0.2. For such spins, and for mass accretion rates inferred from collapsar simulations, we show that our semianalytic model reproduces the energetics of typical GRB jets,L_jet∼ 10⁵⁰erg s⁻¹. We show that our semianalytic model reproduces the nearly constant power of typical GRB jets. If the MAD onset is delayed, this allows powerful jets at the high end of the GRB luminosity distribution,L_jet∼ 10⁵²erg s⁻¹, but the final spin remains low,a≲ 0.3. These results are consistent with the low spins inferred from gravitational wave detections of binary BH mergers. In a companion paper by Gottlieb et al., we use GRB observations to constrain the natal BH spin to bea≃ 0.2. 

Abstract Models invoking magnetic reconnection as the particle acceleration mechanism within relativistic jets often adopt a gradual energy dissipation profile within the jet. However, such a profile has yet to be reproduced in first-principles simulations. Here we perform a suite of 3D general relativistic magnetohydrodynamic simulations of post–neutron star merger disks with an initially purely toroidal magnetic field. We explore the variations in both the microphysics (e.g., nuclear recombination, neutrino emission) and system parameters (e.g, disk mass). In all of our simulations, we find the formation of magnetically striped jets. The stripes result from the reversals in the poloidal magnetic flux polarity generated in the accretion disk. The simulations display large variations in the distributions of stripe duration,τ, and power, 〈P_Φ〉. We find that more massive disks produce more powerful stripes, the most powerful of which reaches 〈P_Φ〉 ∼ 10⁴⁹erg s⁻¹atτ∼ 20 ms. The power and variability that result from the magnetic reconnection of the stripes agree with those inferred in short-duration gamma-ray bursts. We find that the dissipation profile of the cumulative energy is roughly a power law in both radial distance,z, andτ, with a slope in the range of ∼1.7–3; more massive disks display larger slopes. 

Abstract The recent detections of the ∼10 s longγ-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≳ 2.8M_⊙, 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≳ 0.1M_⊙), which form for large binary mass ratiosq≳ 1.2 in BNS orq≲ 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 $\dot{M}\sim t^{-2}$, establishing the EE decay. Two scenarios are plausible for short cbGRBs. They can be powered by BHs with less massive disks, which form for otherqvalues. Alternatively, for binaries withM_tot≲ 2.8M_⊙, 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. 

Abstract The ongoing LIGO–Virgo–KAGRA observing run O4 provides an opportunity to discover new multimessenger events, including binary neutron star (BNS) mergers such as GW170817 and the highly anticipated first detection of a multimessenger black hole–neutron star (BH–NS) merger. While BNS mergers were predicted to exhibit early optical emission from mildly relativistic outflows, it has remained uncertain whether the BH–NS merger ejecta provides the conditions for similar signals to emerge. We present the first modeling of early near-ultraviolet/optical emission from mildly relativistic outflows in BH–NS mergers. Adopting optimal binary properties, a mass ratio ofq= 2, and a rapidly rotating BH, we utilize numerical relativity and general relativistic magnetohydrodynamic (GRMHD) simulations to follow the binary’s evolution from premerger to homologous expansion. We use an M1 neutrino transport GRMHD simulation to self-consistently estimate the opacity distribution in the outflows and find a bright near-ultraviolet/optical signal that emerges due to jet-powered cocoon cooling emission, outshining the kilonova emission at early time. The signal peaks at an absolute magnitude of ∼−15 a few hours after the merger, longer than previous estimates, which did not consider the first principles–based jet launching. By late 2024, the Rubin Observatory will have the capability to track the entire signal evolution or detect its peak up to distances of ≳1 Gpc. In 2026, ULTRASAT will conduct all-sky surveys within minutes, detecting some of these events within ∼200 Mpc. The BH–NS mergers with higher mass ratios or lower BH spins would produce shorter and fainter signals. 

Abstract We present the first numerical simulations that track the evolution of a black hole–neutron star (BH–NS) merger from premerger tor≳ 10¹¹cm. The disk that forms after a merger of mass ratioq= 2 ejects massive disk winds (3–5 × 10⁻²M_⊙). We introduce various postmerger magnetic configurations and find that initial poloidal fields lead to jet launching shortly after the merger. The jet maintains a constant power due to the constancy of the large-scale BH magnetic flux until the disk becomes magnetically arrested (MAD), where the jet power falls off asL_j∼t⁻². All jets inevitably exhibit either excessive luminosity due to rapid MAD activation when the accretion rate is high or excessive duration due to delayed MAD activation compared to typical short gamma-ray bursts (sGRBs). This provides a natural explanation for long sGRBs such as GRB 211211A but also raises a fundamental challenge to our understanding of jet formation in binary mergers. One possible implication is the necessity of higher binary mass ratios or moderate BH spins to launch typical sGRB jets. For postmerger disks with a toroidal magnetic field, dynamo processes delay jet launching such that the jets break out of the disk winds after several seconds. We show for the first time that sGRB jets with initial magnetizationσ₀> 100 retain significant magnetization (σ≫ 1) atr> 10¹⁰cm, emphasizing the importance of magnetic processes in the prompt emission. The jet–wind interaction leads to a power-law angular energy distribution by inflating an energetic cocoon whose emission is studied in a companion paper. 

Abstract Collapsing stars constitute the main black hole (BH) formation channel, and are occasionally associated with the launch of relativistic jets that powerγ-ray bursts (GRBs). Thus, collapsars offer an opportunity to infer the natal (before spin-up/down by accretion) BH spin directly from observations. We show that once the BH saturates with a large-scale magnetic flux, the jet power is dictated by the BH spin and mass accretion rate. Core-collapse simulations by Halevi et al. and GRB observations favor stellar density profiles that yield an accretion rate of $\dot{m}\approx {10}^{-2}{M}_{\odot }{\mathrm{s}}^{-1}$, weakly dependent on time. This leaves the spin as the main factor that governs the jet power. By comparing the jet power to characteristic GRB luminosities, we find that the majority of BHs associated with jets are likely born slowly spinning with a dimensionless spin ofa≃ 0.2, ora≃ 0.5 for wobbling jets, with the main uncertainty originating in the unknownγ-ray radiative efficiency. This result could be applied to the entire core-collapse BH population, unless an anticorrelation between the stellar magnetic field and angular momentum is present. In a companion paper, Jacquemin-Ide et al., we show that regardless of the natal spin, the extraction of BH rotational energy leads to spin-down toa≲ 0.2, consistent with gravitational-wave observations. We verify our results by performing the first 3D general-relativistic magnetohydrodynamic simulations of collapsar jets with characteristic GRB energies, powered by slowly spinning BHs. We find that jets of typical GRB power struggle to escape from the star, providing the first numerical indication that many jets fail to generate a GRB. 

Abstract Long-duration gamma-ray bursts (lGRBs) originate in relativistic collimated outflows—jets—that drill their way out of collapsing massive stars. Accurately modeling this process requires realistic stellar profiles for the jets to propagate through and break out of. Most previous studies have used simple power laws or pre-collapse models for massive stars. However, the relevant stellar profile for lGRB models is in fact that of a star after its core has collapsed to form a compact object. To self-consistently compute such a stellar profile, we use the open-source code GR1D to simulate the core-collapse process for a suite of low-metallicity rotating massive stellar progenitors that have undergone chemically homogeneous evolution. Our models span a range of zero-age main-sequence (ZAMS) masses:M_ZAMS= 13, 18, 21, 25, 35, 40, and 45M_☉. All of these models, at the onset of core-collapse, feature steep density profiles,ρ∝r^−α, withα≈ 2.5, which would result in jets that are inconsistent with lGRB observables. We follow the collapses of four of the seven models until they form black holes (BHs) and the other three models until they form proto-neutron stars (PNSs). We find, across all models, that the density profile outside the newly formed BH or PNS is well represented by a flatter power law withα≈ 1.35–1.55. Such flat density profiles are conducive to the successful formation and breakout of BH-powered jets and are, in fact, required to reproduce observable properties of lGRBs. Future models of lGRBs should be initialized with shallower post-collapse stellar profiles, like those presented here, instead of the much steeper pre-collapse profiles that are typically used.