arXiv subjects

Jonatan Jacquemin-Ide

Publications and source records attributed to Jonatan Jacquemin-Ide.

At least 19 recordsLinked to original sources

arXiv2026

Jets from Scratch: Dynamo-Generated Poloidal Magnetic Fields in 3D Collapsar Simulations

The origin of the large-scale poloidal magnetic field required to power relativistic jets in collapsars remains uncertain. While such a field may be inherited during PNS collapse, the efficiency of this process is unclear, motivating an in situ mechanism to generate poloidal fields out of the predominantly toroidal fields produced by stellar differential rotation. We present the first 3D general-relativistic magnetohydrodynamic collapsar simulations initialized with toroidal magnetic field profiles that closely follows those of pre-collapse stellar models. As the toroidal field in the disk becomes dynamically important, it seeds the dynamo, producing coherent poloidal magnetic loops that appear at $\sim \mathcal{O}(100)$ gravitational radii and are then advected inward along paths that may deviate from the disk midplane. The resulting poloidal fields thread the black hole (BH) and launch highly variable, wobbling relativistic jets on timescales of order seconds, with the onset depending on the initial magnetic field and the plasma circularization radius. Although the jets are highly variable and misaligned with the BH spin axis, they sustain $\gtrsim 10^{50}\,\mathrm{erg\,s^{-1}}$, comparable to that inferred for long gamma-ray bursts (LGRB). We identify magnetic-flux inversions driven by the stochastic dynamo, leading to the formation of striped jets that could be imprinted in LGRB light curves. These results demonstrate that accretion-disk dynamos provide a robust pathway for jet production in collapsars across a broad range of progenitors.

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arXiv2026

Evidence for Low Universal Equilibrium Black Hole Spin in Luminous Magnetically Arrested Disks

Relativistic collimated outflows, or jets, provide a crucial mode of active galactic nucleus feedback. Although jets extract their energy from the black hole (BH) rotation, their effect on the BH spin is poorly understood. Because the spin controls radiative and mechanical BH feedback, lack of first-principles models for spin evolution limits our ability to interpret observations, including the recent LIGO-Virgo-KAGRA spin constraints. Particularly important are luminous disks, which rapidly grow and strongly torque their BHs. Jetless and weakly magnetized standard luminous disks spin up their BHs to near-maximum spin, $a_{eq,NT}=0.998$. However, sufficient large-scale vertical magnetic flux can cause the inner disk to enter a magnetically arrested disk (MAD) state, whose jets can efficiently extract BH rotational energy and significantly spin down the BH. Lowell et al. (2024) found that nonradiative, thick MADs spin down their BHs to very low $a_{eq,MAD}^{thick}=0.07$. Their analytic model predicted that luminous, thin MADs also spin down their BHs to low $a_{eq,MAD}^{thin}\sim0.3\text{-}0.5$. To test this prediction, we perform 3D general relativistic (radiation) magnetohydrodynamic (GR(R)MHD) simulations of MADs across a wide range of BH spin ($-0.9\le{}a\le0.99$) and disk thickness ($0.03\le{}h/r\le0.3$, which corresponds to Eddington ratio, $0.35\le{}\dot{m}/\dot{m}_{Edd}\le\infty$). We find that luminous, thin MADs ($0.03\le{}h/r\le0.1$) efficiently spin down their BHs to a low universal equilibrium spin value, $a_{eq,MAD}^{thin}\approx0.3$: a maximally spinning BH ($a=1$) spins down to $a=0.5$ after accreting just $25\%$ of its initial mass. Our results follow quadratic convergence, $a_{eq,MAD}^{fit}\simeq0.3-2.7(h/r)^2\to0.3$ as $h/r\to0$, which we attribute to the aggressive cooling that renders disk thermodynamics irrelevant and magnetic forces insensitive to thermal $h/r$.

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arXiv2025

Demystifying flux eruptions: Magnetic flux transport in magnetically arrested disks

Magnetically arrested disks (MADs) are a compelling model for explaining variability in low-luminosity active galactic nuclei (AGN), including horizon-scale outbursts like those observed in Sagittarius A*. MADs experience powerful flux eruptions-episodic ejections of magnetic flux from the black hole horizon-that may drive the observed luminosity variations. In this work, we develop and validate a new formalism describing large-scale magnetic field transport in general relativistic magnetohydrodynamic simulations of MADs with geometrical thicknesses of $h/R=0.1$ and $h/R=0.3$. We introduce a net flux transport velocity, $v_Φ$, which accounts for both advective and diffusive processes. We show that MADs maintain a statistical quasi-steady state where advection and diffusion nearly balance. Flux eruptions appear as small deviations from this equilibrium, with $v_Φ/V_k\ll1$, where $V_k$ is the local Keplerian velocity. Using this framework, we analytically derive a recurrence timescale for flux eruptions, $t_{\rm rec}\sim1500\, r_g/c$. This timescale closely matches simulation results. The smallness of $v_Φ$ explains the long recurrence times of flux eruptions compared to other system timescales. We also take a closer look at the diffusion of the magnetic field by performing the first measurement of turbulent resistivity in MADs. We then estimate the turbulent magnetic Prandtl number, defined as the ratio of turbulent viscosity to turbulent resistivity. We find $\mathcal{P}_m\sim3$, consistent with shearing-box simulations of magneto rotational instability-driven turbulence. While flux eruptions excite large-scale non-axisymmetric modes and locally enhance turbulent resistivity, magnetic field diffusion is dominated by smaller-scale turbulent motions. These results provide new insight into the nature of AGN variability and the fundamental physics of magnetic field transport.

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arXiv2025

Magnetically-Driven Neutron-Rich Ejecta Unleashed: Global 3D Neutrino-General Relativistic Magnetohydrodynamic Simulations of Collapsars Probe the Conditions for r-process Nucleosynthesis

Collapsars - rapidly rotating stellar cores that form black holes - can power gamma-ray bursts (GRBs) and are proposed to be key contributors to the production of heavy elements in the Universe via the rapid neutron capture process ($r$-process). Previous neutrino-transport collapsar simulations have been unable to unbind neutron-rich material from the disk. However, these simulations have not included sufficiently strong magnetic fields and the black hole (BH), both of which are essential for launching mass outflows. We present $nu$H-AMR, a novel neutrino-transport general relativistic magnetohydrodynamic ($ν$GRMHD) code, which we use to perform the first 3D global $ν$GRMHD collapsar simulations. We find a self-consistent formation of a weakly magnetized dense accretion disk, which has sufficient time to neutronize. Eventually, substantial magnetic flux accumulates near the BH, becomes dynamically important, leads to a magnetically arrested disk (MAD), and unbinds some of the neutron-rich material. However, the strong flux also hinders accretion, lowers density, and increases neutrino cooling timescale, which prevents further disk neutronization. Typical collapsar progenitors with mass accretion rates, $\dot{M} \sim 0.1-1 M_\odot/\rm{s}$, do not produce significant neutron-rich ($Y_\text{e} < 0.25$) ejecta. However, we find that MADs at higher mass accretion rates, $\dot{M} \gtrsim \text{few}\, M_\odot/\rm{s}$ (e.g., for more centrally concentrated progenitors), can unbind $M_\text{ej}\lesssim{}M_\odot$ of neutron-rich ejecta. The outflows inflate a shocked cocoon that mixes with the infalling neutron-poor stellar gas and raises the final outflow $Y_\text{e}$; however, the final $r$-process yield may be determined earlier at the point of neutron capture freeze-out. Future work will explore under what conditions more typical collapsar engines become $r$-process factories.

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arXiv2025

Black hole spectral states revealed in GRMHD simulations with texture memory accelerated cooling

X-ray binaries (XRBs) display spectral state transitions that are accompanied by substantial changes in the hardness, luminosity, and structure of the accretion flow. We developed a GPU-accelerated cooling toolkit for general relativistic magnetohydrodynamic (GRMHD) simulations of accreting black holes that uses texture memory for fast retrieval of pre-computed values. The toolkit incorporates bremsstrahlung, synchrotron, inverse Compton radiation and Coulomb collision processes. We implemented our toolkit into a GRMHD code and used it to simulate a magnetically arrested disk in the context of the XRB low/hard state around a Kerr black hole. We explored the mass accretion rate in the $\sim (10^{-6}-0.3) \dot{M}_{\rm Edd}$ range, where $\dot{M}_{\rm Edd}$ is the Eddington accretion rate. Our simulations reveal that for low accretion rates ($\dot{M} \lesssim 0.01 \dot{M}_{\rm Edd}$), the flow settles into a geometrically thick, low-density, two-temperature hot accretion flow. At higher accretion rates, the flow turns into a cold single-temperature thin disk at $r_{\rm in} \gtrsim 50 r_g$. Inside, the disk breaks up into single-temperature thin filaments embedded into a two-temperature hot thick flow. Our GPU texture memory accelerated cooling prescription is $3-5$ times faster than the standard radiation M1 closure methods, and $\sim5$ times faster than storing the lookup table in global memory.

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arXiv2025

Black Hole Spin-down in Collapsars in 3D Neutrino Transport GRMHD Simulations

Collapsars -- massive stars whose cores promptly collapse into black holes (BHs) -- can power long-duration gamma-ray bursts (LGRBs) via relativistic, collimated, electromagnetically-driven outflows, or jets. Their power depends on the BH magnetic field strength and spin. To survive the infalling stellar material, jets need the central BH to attain dynamically important magnetic fields that can suppress the mass inflow and lead to a magnetically arrested disk (MAD). Previous work found that non-radiative MADs can spin down their BHs to an equilibrium spin, $a_{\rm eq}^\text{nr}=0.035-0.07$. Such low spins result in extremely low power jets that may struggle to escape out of the star. However, the dense and hot collapsar disks emit neutrinos that cool the disk, reduce its thickness, and increase the angular momentum supply to the BH. Using 3D two-moment neutrino-transport general relativistic magnetohydrodynamic simulations, we show for the first time that successful collapsar jets powered by neutrino-cooled disks still rapidly spin down their BHs, although to a higher $a_{\rm eq}\approx 0.13$. This value is consistent with LIGO/Virgo/KAGRA inferred spins, is $2-4$x higher than for non-radiative MADs, and results in $4-16$x more powerful LGRB jets, which are more capable of drilling out of the progenitor star. This value of $a_{\rm eq}$ holds across a wide range of progenitor structures and mass accretion rates, $\dot{m} \sim(0.1-10)M_{\odot}/\rm{s}$. We find that for typical LGRB durations, $t\gtrsim30$~s, such BHs consume sufficient mass to reach $a_{\rm eq} \approx 0.13$ by LGRB's end. However, shorter or lower-$\dot{m}$ LGRBs can leave behind more rapidly spinning BHs.

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arXiv2025

H-AMR FORGE'd in FIRE I: Magnetic state transitions, jet launching and radiative emission in super-Eddington, highly magnetized quasar disks formed from cosmological initial conditions

Quasars are powered by supermassive black hole (SMBH) accretion disks, yet standard disk models are inconsistent with many quasar observations. Recently, Hopkins et al. (2024) simulated the formation of a quasar disk feeding a SMBH of mass $M=1.3\times10^7\,M_\odot$ in a host galaxy that evolved from cosmological initial conditions. The disk had surprisingly strong toroidal magnetic fields that supported it vertically from gravity and powered fast accretion. What radiation and feedback can such a system produce? To answer this, we must follow the gas to the event horizon. For this, we interpolated the accretion system onto the grid of the general-relativistic radiation magnetohydrodynamics code H-AMR and performed 3D simulations with BH spins $a=0$ and $a=0.9375$. This remapping generates spurious magnetic monopoles, which we erase using a novel divergence cleaning approach. Despite the toroidal magnetic field's dominance at large radii, vertical magnetic flux builds up at the event horizon. This causes a magnetic state transition within the inner $200$ gravitational radii of the disk, where net vertical magnetic flux begins dominating the accretion flow. This powers strong winds and, if the BH spins, relativistic jets that can spin-down the BH within $5-10\,{\rm Myrs}$. Sometimes, vertical magnetic fields of opposite sign reach the BH, causing polarity inversion events that briefly destroy the jets and, possibly, the X-ray corona. The disk powers accretion at rates $5\times$ the Eddington limit, which can double the BH mass in $5-10\,{\rm Myrs}$. When $a=0.9375$ ($a=0$), the energy in outflows and radiation equals about $60\%$ ($10\%$) and $100\%$ ($3\%$) of the accreted rest mass energy, respectively. Much of the light escapes in cool, extended $\gtrsim1300\,{\rm au}$ photospheres, consistent with quasar microlensing and the ``big blue bump'' seen in spectral energy distributions.

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arXiv2024

Nozzle Shocks, Disk Tearing and Streamers Drive Rapid Accretion in 3D GRMHD Simulations of Warped Thin Disks

The angular momentum of gas feeding a black hole (BH) is typically misaligned with respect to the BH spin, resulting in a tilted accretion disk. Rotation of the BH drags the surrounding space-time, manifesting as Lense-Thirring torques that lead to disk precession and warping. We study these processes by simulating a thin ($H/r=0.02$), highly tilted ($\mathcal{T}=65^\circ$) accretion disk around a rapidly rotating ($a=0.9375$) BH at extremely high resolutions, which we performed using the general-relativistic magnetohydrodynamic (GRMHD) code H-AMR. The disk becomes significantly warped and continuously tears into two individually precessing sub-disks. We find that mass accretion rates far exceed the standard $α$-viscosity expectations. We identify two novel dissipation mechanisms specific to warped disks that are the main drivers of accretion, distinct from the local turbulent stresses that are usually thought to drive accretion. In particular, we identify extreme scale height oscillations that occur twice an orbit throughout our disk. When the scale height compresses, `nozzle' shocks form, dissipating orbital energy and driving accretion. Separate from this phenomenon, there is also extreme dissipation at the location of the tear. This leads to the formation of low-angular momentum `streamers' that rain down onto the inner sub-disk, shocking it. The addition of low angular momentum gas to the inner sub-disk causes it to rapidly accrete, even when it is transiently aligned with the BH spin and thus unwarped. These mechanisms, if general, significantly modify the standard accretion paradigm. Additionally, they may drive structural changes on much shorter timescales than expected in $α$-disks, potentially explaining some of the extreme variability observed in active galactic nuclei.

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arXiv2024

Winds and Disk Turbulence Exert Equal Torques on Thick Magnetically Arrested Disks

The conventional accretion disk lore is that magnetized turbulence is the principal angular momentum transport process that drives accretion. However, when dynamically important large-scale magnetic fields thread an accretion disk, they can produce mass and angular momentum outflows, known as winds, that also drive accretion. Yet, the relative importance of turbulent and wind-driven angular momentum transport is still poorly understood. To probe this question, we analyze a long-duration ($1.2 \times 10^5 r_{\rm g}/c$) simulation of a rapidly rotating ($a=0.9$) black hole feeding from a thick ($H/r\sim0.3$), adiabatic, magnetically arrested disk (MAD), whose dynamically important magnetic field regulates mass inflow and drives both uncollimated and collimated outflows (i.e., winds and jets, respectively). By carefully disentangling the various angular momentum transport processes within the system, we demonstrate the novel result that disk winds and disk turbulence both extract roughly equal amounts of angular momentum from the disk. We find cumulative angular momentum and mass accretion outflow rates of $\dot{L}\propto r^{0.9}$ and $\dot{M}\propto r^{0.4}$, respectively. This result suggests that understanding both turbulent and laminar stresses is key to understanding the evolution of systems where geometrically thick MADs can occur, such as the hard state of X-ray binaries, low-luminosity active galactic nuclei, some tidal disruption events, and possibly gamma-ray bursts.

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arXiv2024

Rapid Black Hole Spin-down by Thick Magnetically Arrested Disks

Black hole (BH) spin can play an important role in galaxy evolution by controlling the amount of energy and momentum ejected from near the BH into the surroundings. We focus on radiatively-inefficient and geometrically-thick magnetically-arrested disks (MADs) that can launch strong BH-powered jets. With an appropriately chosen adiabatic index, these systems can describe either the low-luminosity or highly super-Eddington BH accretion regimes. Using a suite of 3D general relativistic magnetohydrodynamic (GRMHD) simulations, we find that for any initial spin, a MAD rapidly spins down the BH to the equilibrium spin of $0< a_{\rm eq} \lesssim 0.1$, very low compared to $a_{\rm eq} = 1$ for the standard thin luminous (Novikov-Thorne) disks. This implies that rapidly accreting (super-Eddington) BHs fed by MADs tend to lose most of their rotational energy to magnetized relativistic outflows. In a MAD, a BH only needs to accrete $20\%$ of its own mass to spin down from $a=1$ to $a=0.2$. We construct a semi-analytic model of BH spin evolution in MADs by taking into account the torques on the BH due to both the hydrodynamic disk and electromagnetic jet components, and find that the low value of $a_{\rm eq} $ is due to both the jets slowing down the BH rotation and the disk losing a large fraction of its angular momentum to outflows. Our results have crucial implications for how BH spins evolve in active galaxies and other systems such as collapsars, where BH spin-down timescale can be short enough to significantly affect the evolution of gamma-ray emitting BH-powered jets.

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arXiv2024

Impact of the disk magnetization on MHD disk wind signature

Observation of blue-shifted X-ray absorption lines indicates the presence of wind from the accretion disk in X-ray binaries. Magnetohydrodynamic (MHD) driving is one of the possible wind launching mechanisms. Recent theoretical development makes magnetic accretion-ejection self-similar solutions much more generalized, and wind can be launched even at much lower magnetization compared to equipartition value, which was the only possibility beforehand. Here, we model the transmitted spectra through MHD driven photoionized wind - models which have different values of magnetizations. We investigate the possibility of detecting absorption lines by the upcoming instruments XRISM and Athena. Attempts are made to find the robustness of the method of fitting asymmetric line profiles by multiple Gaussians. We use photoionization code XSTAR to simulate the transmitted model spectra. Fake observed spectra are finally produced by convolving model spectra with instruments' responses. Since the line asymmetries are apparent in the convolved spectra as well, this can be used as an observable diagnostic to fit for, in future XRISM and Athena spectra. We demonstrate some amount of rigor in assessing the equivalent widths of the major absorption lines, including the Fe XXVI Ly$α$ doublets which can be clearly distinguished in the superior quality, future high resolution spectra. Disk magnetization becomes another crucial MHD variable that can significantly alter the absorption line profiles. Low magnetization pure MHD outflow models are dense enough to be observed by the existing or upcoming instruments. Thus these models become simpler alternatives to MHD-thermal models. Fitting with multiple Gaussians is a promising method to handle asymmetric line profiles, as well as the Fe XXVI Ly$α$ doublets.

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arXiv2023

From Feast to Famine: A Systematic Study of Accretion onto Oblique Pulsars with 3D GRMHD Simulations

Disk-fed accretion onto neutron stars can power a wide range of astrophysical sources ranging from X-ray binaries, to accretion powered millisecond pulsars, ultra-luminous 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 anti-parallel 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.

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arXiv2023

Magnetorotational dynamo can generate large-scale vertical magnetic fields in 3D GRMHD simulations of accreting black holes

Jetted astrophysical phenomena with black hole (BH) engines, including binary mergers, jetted tidal disruption events, and X-ray binaries, require a large-scale vertical magnetic field for efficient jet formation. However, a dynamo mechanism that could generate these crucial large-scale magnetic fields has not been identified and characterized. We have employed 3D global general relativistic magnetohydrodynamical (MHD) simulations of accretion disks to quantify, for the first time, a dynamo mechanism that generates large-scale magnetic fields. This dynamo mechanism primarily arises from the nonlinear evolution of the magnetorotational instability (MRI). In this mechanism, large non-axisymmetric MRI-amplified shearing wave modes, mediated by the axisymmetric azimuthal magnetic field, generate and sustain the large-scale vertical magnetic field through their nonlinear interactions. We identify the advection of magnetic loops as a crucial feature, transporting the large-scale vertical magnetic field from the outer regions to the inner regions of the accretion disk. This leads to a larger characteristic size of the, now advected, magnetic field when compared to the local disk height. We characterize the complete dynamo mechanism with two timescales: one for the local magnetic field generation, $t_{\rm g}$, and one for the large-scale scale advection, $t_{\rm adv}$. Whereas the dynamo we describe is nonlinear, we explore the potential of linear mean field models to replicate its core features. Our findings indicate that traditional $α$-dynamo models, often computed in stratified shearing box simulations, are inadequate and that the effective large-scale dynamics is better described by the shear current effects or stochastic $α$-dynamos.

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arXiv2023

Jets with a Twist: Emergence of FR0 Jets in 3D GRMHD Simulation of Zero Angular Momentum Black Hole Accretion

Spinning supermassive black holes (BHs) in active galactic nuclei (AGN) magnetically launch relativistic collimated outflows, or jets. Without angular momentum supply, such jets are thought to perish within $3$ orders of magnitude in distance from the BH, well before reaching kpc-scales. We study the survival of such jets at the largest scale separation to date, via 3D general relativistic magnetohydrodynamic simulations of rapidly spinning BHs immersed into uniform zero-angular-momentum gas threaded by weak vertical magnetic field. We place the gas outside the BH sphere of influence, or the Bondi radius, chosen much larger than the BH gravitational radius, $R_\text{B}=10^3R_\text{g}$. The BH develops dynamically-important large-scale magnetic fields, forms a magnetically-arrested disk (MAD), and launches relativistic jets that propagate well outside $R_\text{B}$ and suppress BH accretion to $1.5\%$ of the Bondi rate, $\dot{M}_\text{B}$. Thus, low-angular-momentum accretion in the MAD state can form large-scale jets in Fanaroff-Riley (FR) type I and II galaxies. Subsequently, the disk shrinks and exits the MAD state: barely a disk (BAD), it rapidly precesses, whips the jets around, globally destroys them, and lets $5-10\%$ of $\dot{M}_\text{B}$ reach the BH. Thereafter, the disk starts rocking back and forth by angles $90-180^\circ$: the rocking accretion disk (RAD) launches weak intermittent jets that spread their energy over a large area and suppress BH accretion to $\lesssim 2 \% ~ \dot{M}_\text{B}$. Because BAD and RAD states tangle up the jets and destroy them well inside $R_\text{B}$, they are promising candidates for the more abundant, but less luminous, class of FR0 galaxies.

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arXiv2023

Angular Momentum Loss During Stable Mass Transfer onto a Compact object: the Effect of Mass Loss via Accretion Disk Winds

We use an analytic framework to calculate the evolution of binary orbits under a physically-motivated model that accounts for angular momentum loss associated with winds from an accretion disk around the compact objected accretor. Our prescription considers wind mass ejection from the surface of an accretion disk, accounting for a radial mass-loss dependence across the disk surface. We compare this to the standard prescription of angular momentum loss associated with isotropic mass loss from the vicinity of the accretor. The angular momentum loss from a disk-wind is always larger. For mass ratios, $q$, between $2$--$10$, angular momentum loss via a disk wind is $\simeq3$--$40$ times greater than the standard prescription. For the majority of mass ratios and disk properties, accounting for the disk wind can result in considerably smaller orbital separations compared to the standard formalism; the differences being $\simeq 60\%$ depending on how long the effect is integrated for. We conclude that it is important to consider the effects of angular momentum loss from a disk wind when evolving binary orbits.

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arXiv2023

Large-scale Evolution of Seconds-long Relativistic Jets from Black Hole-Neutron Star Mergers

We present the first numerical simulations that track the evolution of a black hole-neutron star (BH-NS) merger from pre-merger to $r\gtrsim10^{11}\,{\rm cm}$. The disk that forms after a merger of mass ratio $q=2$ ejects massive disk winds ($3-5\times10^{-2}\,M_{\odot}$). We introduce various post-merger 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 as $L_j\sim t^{-2}$. 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 post-merger 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 magnetization $σ_0>100$ retain significant magnetization ($σ\gg1$) at $r>10^{10}\,{\rm cm}$, 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.

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arXiv2023

Hours-long Near-UV/Optical Emission from Mildly Relativistic Outflows in Black Hole-Neutron Star Mergers

The ongoing LIGO-Virgo-KAGRA observing run O4 provides an opportunity to discover new multi-messenger events, including binary neutron star (BNS) mergers such as GW170817, and the highly anticipated first detection of a multi-messenger 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 of $q=2$ and a rapidly rotating BH, we utilize numerical relativity and general relativistic magnetohydrodynamic (GRMHD) simulations to follow the binary's evolution from pre-merger 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 $\sim -15$ 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 $\gtrsim1$ Gpc. In 2026, ULTRASAT will conduct all-sky surveys within minutes, detecting some of these events within $\sim 200$ Mpc. The BH-NS mergers with higher mass ratios or lower BH spins would produce shorter and fainter signals.

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arXiv2023

Jetted and Turbulent Stellar Deaths: New LVK-Detectable Gravitational Wave Sources

Upcoming LIGO/Virgo/KAGRA (LVK) observing runs are expected to detect a variety of inspiralling gravitational-wave (GW) events, that come from black-hole and neutron-star binary mergers. Detection of non-inspiral GW sources is also anticipated. We report the discovery of a new class of non-inspiral GW sources - the end states of massive stars - that can produce the brightest simulated stochastic GW burst signal in LVK bands known to date, and could be detectable in the LVK run A+. Some dying massive stars launch bipolar relativistic jets, which inflate a turbulent energetic bubble - cocoon - inside of the star. We simulate such a system using state-of-the-art 3D general-relativistic magnetohydrodynamic simulations and show that these cocoons emit quasi-isotropic GW emission in the LVK band, $\sim 10-100$ Hz, over a characteristic jet activity timescale, $\sim 10-100$ s. Our first-principles simulations show that jets exhibit a wobbling behavior, in which case cocoon-powered GWs might be detected already in LVK run A+, but it is more likely that these GWs will be detected by the third generation GW detectors with estimated rate of $ \sim 10 $ events/year. The detection rate drops to $ \sim 1\% $ of that value if all jets were to feature a traditional axisymmetric structure instead of a wobble. Accompanied by electromagnetic emission from the energetic core-collapse supernova and the cocoon, we predict that collapsars are powerful multi-messenger events.

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