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1. On the Jet–Ejecta Interaction in 3D GRMHD Simulations of a Binary Neutron Star Merger Aftermath

   https://doi.org/10.3847/2041-8213/ac7728  

   Gottlieb, Ore; Moseley, Serena; Ramirez-Aguilar, Teresita; Murguia-Berthier, Ariadna; Liska, Matthew; Tchekhovskoy, Alexander (June 2022, The Astrophysical Journal Letters)

   Abstract Short γ -ray burst (sGRB) jets form in the aftermath of a neutron star merger, drill through disk winds and dynamical ejecta, and extend over four to five orders of magnitude in distance before breaking out of the ejecta. We present the first 3D general-relativistic magnetohydrodynamic sGRB simulations to span this enormous scale separation. They feature three possible outcomes: jet+cocoon, cocoon, and neither. Typical sGRB jets break out of the dynamical ejecta if (i) the bound ejecta’s isotropic equivalent mass along the pole at the time of the BH formation is ≲10 −4 M ⊙ , setting a limit on the delay time between the merger and BH formation, otherwise, the jets perish inside the ejecta and leave the jet-inflated cocoon to power a low-luminosity sGRB; (ii) the postmerger remnant disk contains a strong large-scale vertical magnetic field, ≳10 15 G; and (iii) if the jets are weak (≲10 50 erg), the ejecta’s isotropic equivalent mass along the pole must be small (≲10 −2 M ⊙ ). Generally, the jet structure is shaped by the early interaction with disk winds rather than the dynamical ejecta. As long as our jets break out of the ejecta, they retain a significant magnetization (≲1), suggesting that magnetic reconnection is a fundamental property of sGRB emission. The angular structure of the outflow isotropic equivalent energy after breakout consistently features a flat core followed by a steep power-law distribution (slope ≳3), similar to hydrodynamic jets. In the cocoon-only outcome, the dynamical ejecta broadens the outflow angular distribution and flattens it (slope ∼1.5). 

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2. Collapsar Gamma-Ray Bursts Grind Their Black Hole Spins to a Halt

   https://doi.org/10.3847/1538-4357/ad02f0  

   Jacquemin-Ide, Jonatan; Gottlieb, Ore; Lowell, Beverly; Tchekhovskoy, Alexander (January 2024, The Astrophysical Journal)

   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. 

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3. The Jet Opening Angle and Event Rate Distributions of Short Gamma-Ray Bursts from Late-time X-Ray Afterglows

   https://doi.org/10.3847/1538-4357/acf830  

   Rouco Escorial, A.; Fong, W.; Berger, E.; Laskar, T.; Margutti, R.; Schroeder, G.; Rastinejad, J. C.; Cornish, D.; Popp, S.; Lally, M.; et al (November 2023, The Astrophysical Journal)

   Abstract We present a comprehensive study of 29 short gamma-ray bursts (SGRBs) observed ≈0.8−60 days postburst using Chandra and XMM-Newton. We provide the inferred distributions of the SGRB jet opening angles and true event rates to compare against neutron star merger rates. We perform a uniform analysis and modeling of their afterglows, obtaining 10 opening angle measurements and 19 lower limits. We report on two new opening angle measurements (SGRBs 050724A and 200411A) and eight updated values, obtaining a median value of 〈θ_j〉 ≈ 6.°1 [−3.°2, +9.°3] (68% confidence on the full distribution) from jet measurements alone. For the remaining events, we inferθ_j≳ 0.°5–26°. We uncover a population of SGRBs with wider jets ofθ_j≳ 10° (including two measurements ofθ_j≳ 15°), representing ∼28% of our sample. Coupled with multiwavelength afterglow information, we derive a total true energy of 〈E_true,tot〉 ≈ 10⁴⁹–10⁵⁰erg, which is consistent with magnetohydrodynamic jet launching mechanisms. Furthermore, we determine a range for the beaming-corrected event rate of ${\mathfrak{R}}_{\mathrm{true}}\approx 360-1800$Gpc⁻³yr⁻¹, set by the inclusion of a population of wide jets on the low end, and the jet measurements alone on the high end. From a comparison with the latest merger rates, our results are consistent with the majority of SGRBs originating from binary neutron star mergers. However, our inferred rates are well above the latest neutron star–black hole merger rates, consistent with at most a small fraction of SGRBs originating from such mergers. 

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4. Maximal Jet Energy of Gamma-Ray Bursts through the Blandford–Znajek Mechanism

   https://doi.org/10.3847/2041-8213/adaeb8  

   Wu, Zhao-Feng; Damoulakis, Michail; Beniamini, Paz; Giannios, Dimitrios (February 2025, The Astrophysical Journal Letters)

   Abstract Gamma-ray bursts (GRBs) are among the most energetic events in the Universe, driven by relativistic jets launched from black holes (BHs) formed during the collapse of massive stars or after the merger of two neutron stars. The jet power depends on the BH spin and the magnetic flux accreted onto it. In the standard thin disk model, jet power is limited by insufficient magnetic flux, even when the spin approaches maximum possible value. In contrast, the magnetically arrested disk (MAD) state limits jet energy by extracting significant angular momentum, braking BH rotation. We propose a unified model incorporating both standard thin disk and MAD states, identifying a universal curve for jet power per accretion rate as a function of the magnetic flux ratio, ${\Delta }_{\mathrm{eq}}={({\Phi }_{\mathrm{BH}}/{\Phi }_{\mathrm{MAD}})}_{\mathrm{eq}}$, at spin equilibrium. For long GRBs (lGRBs), the model predicts a maximum jet energy of ∼1.5% of the accretion energy, occurring at Δ_eq ∼ 0.4, where the BH equilibrium spin isa ∼ 0.5. Both long and short GRBs are unlikely to be produced by a MAD: for short GRBs, this requires an accreted mass orders of magnitude smaller than that available, while for lGRBs, the narrow progenitor mass distribution challenges the ability to produce the observed broad distribution of jet energies. This framework provides a consistent explanation for both standard and luminous GRBs, emphasizing the critical role of magnetic flux. Both long and short GRBs require magnetic flux distributions that peak around 10²⁷G cm². 

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5. Subphotospheric Emission from Short Gamma-Ray Bursts. II. Signatures of Nonthermal Dissipation in the Multi-messenger Signals

   https://doi.org/10.3847/1538-4357/adbd3e  

   Rudolph, Annika; Tamborra, Irene; Gottlieb, Ore (April 2025, The Astrophysical Journal)

   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. 

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