arXiv subjects

Alan Tsz-Lok Lam

Publications and source records attributed to Alan Tsz-Lok Lam.

At least 19 recordsLinked to original sources

arXiv2026

Gravitational waves from core collapse of rotating very-massive stars: 3D numerical relativity computation

We numerically study the collapse of rotating, very-massive stellar cores with masses of $\approx 200$, $300$, $500$, and $1100M_\odot$ into black holes using both axisymmetric and three-dimensional (3D) numerical relativity. Our results indicate that when the dimensionless spin of the resulting black hole exceeds 0.8, a massive disk consistently forms around it. These massive, compact disks, carrying more than about 10\% of the black hole's mass, are prone to non-axisymmetric deformations that trigger gravitational-wave bursts with frequencies around 10-50 Hz. Such waves could be detected by the Einstein Telescope and Cosmic Explorer, even from sources a few Gpc away. We also summarize the gravitational-wave signals from axisymmetric collapse, which tend to have lower amplitudes and higher frequencies than those caused by non-axisymmetric instabilities.

gr-qc↗

arXiv2026

Explosions from Rotating Very Massive Star Collapses to Black Holes: Effects of Nuclear Burning

We investigate the collapse of rotating very massive and supermassive stellar cores using numerical relativity simulations including an alpha-chain nuclear reaction network and neutrino cooling. Our main survey focuses on newly constructed models with initial core masses of $2 \times {10}^{3}-5\times 10^4M_\odot$. The collapse is triggered either by pair instability in lower-mass cores or by general-relativistic instability in higher-mass cores. We find that higher-mass cores undergo a nearly homologous collapse, whereas lower-mass cores show a more runaway-like collapse because neutrino cooling becomes more efficient at their higher densities and temperatures. Consequently, the black hole formed in lower-mass models initially contains a smaller fraction of the core mass, and disk formation occurs while a larger amount of matter remains outside the black hole. The lower compactness of pair-unstable cores also allows larger dimensionless angular momentum, favoring the formation of rapidly rotating black holes and massive disks. The disk bounce drives mass ejection with ejecta masses of order $10-10^3M_\odot$ and kinetic energies of order $10^{53}-10^{55}\,\mathrm{erg}$. Significant $^{56}$Ni production in the disk-bounce ejecta occurs only in the lowest-mass models. For selected models, we further follow the viscous evolution of the disk and find that viscosity enhances the ejecta mass and kinetic energy. In models with $\lesssim10^4M_\odot$, the viscosity-driven ejecta can originate from disk matter that has reached nuclear statistical equilibrium and can therefore become rich in $^{56}$Ni. These results suggest that rotating very massive star collapses can produce massive, energetic ejecta and, for sufficiently low core masses, substantial iron-group elements.

astro-ph.HE↗

arXiv2026

Exact Mass Conservation in Binary Neutron Star Merger Simulations

A long-standing problem in the simulation of neutron star spacetimes is the treatment of vacuum regions outside the stars. The use of an artificial low-density atmosphere is a common robust approach within Eulerian hydrodynamics that, however, introduces baryon-mass violation even with conservative numerical schemes. We propose a simple numerical algorithm that ensures exact mass conservation by means of an appropriate local rescaling of the atmosphere. The scheme is combined with a low-order flux correction and it can be further augmented by a pseudo-vacuum treatment that enforces strict vacuum in the outer regions far from the central objects. We demonstrate the effectiveness of these vacuum treatments with binary neutron star mergers simulations spanning multiple orbits and the postmerger phase, and including a microphysical equation of state. The rescaling algorithm guarantees mass and electron number conservation to round-off precision. The pseudo-vacuum treatment shows slightly larger but approximately constant violations and can improve the computation of fast tail ejecta as well as provide convergent gravitational waves of quality comparable to the standard atmosphere. Overall, results from different atmosphere treatments and a two-code comparison suggest that current computations of gravitational waves and (dynamical) ejecta in the presence of an artifical atmosphere are robust, provided that conservative adaptive mesh refinement with flux correction is employed.

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arXiv2026

Donutization Inside Neutron Stars: Shell-Localized Scalar Fields

Heavy scalar fields ($m_ϕ\gtrsim10^{-9}$ eV) in scalar-tensor gravity are expected to be hidden from neutron-star observations because their Compton wavelength is sub-stellar. We show that neutron stars can nevertheless scalarize by forming a shell-localized profile, suppressed at their center and exterior but peaked in their interior. This \emph{donutization} reshapes the effective equation of state, making hadronic stars mimic quark-star mass-radius behavior or hybrid-star behavior with split stable branches, and breaks the $I$--$Q$ relation, while remaining hidden from binary pulsar observations.

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arXiv2026

Accretion-powered flares from black hole-disk collisions in galactic nuclei

Black hole impacts on accretion disks in galactic nuclei can power luminous transients, but predicting their observable signatures is challenging because the post-collision flow is highly time-dependent and inhomogeneous. We present a radiative post-processing framework for relativistic hydrodynamics simulations of black hole-disk collisions. Using physically motivated prescriptions for shock heating, optical depth via an eikonal solver, and photon escape fractions that account for advection trapping and diffusion, we predict light curves and spectral energy distributions over a range of disk densities and collision velocities. Our results indicate that the emission is dominated by the long-lived, highly super-Eddington accretion flow onto the secondary black hole, rather than by cooling of the unbound ejecta. In the parameter range explored, the luminosity can reach several times the Eddington luminosity of the secondary, and the emission is generically dominated by soft X-rays. We find that lower velocity collisions produce brighter flares, while the disk surface density mainly controls spectral evolution: low-density disks typically produce keV-peaked flares with weak spectral evolution, whereas high-density disks show softer early emission and late-time hardening. A depletion-time estimate calibrated to our results suggests characteristic durations of hours to days for intermediate-mass secondaries, and yields $t_{\rm flare} \propto P_{\rm QPE}$. We discuss implications for QPE-like transients and for the SMBH-binary candidate OJ 287.

astro-ph.HE↗

arXiv2026

Convective stability analysis of massive neutron stars formed in binary mergers

We perform fully general-relativistic hydrodynamics simulations of binary neutron star mergers over $100\,\rm ms$ post-merger to investigate the dynamics of remnant massive neutron stars (NSs). Our focus is mainly on the analysis of convective stability and mode characteristics of the massive NSs. We derive stability criteria for hot, differentially rotating relativistic stars that account for both buoyant and rotational restoring forces, and apply them for the first time to the post-merger massive NSs. Our results show no evidence of large-scale convective instability, as both angle-averaged specific entropy and specific angular momentum increase outward within the massive NSs. Rotational effects significantly enhance stability for local regions that would be otherwise unstable by the Schwarzschild criterion. Additionally, our mode analysis of matter fields and gravitational waves reveals no excitation of observable inertial modes after the damping of quadrupolar $f$-modes in the massive NSs, contrasting with previous studies. As in many previous works, we observe the excitation of an $m=1$ one-armed mode. However, we also find that the growth of the $m=1$ mode amplitude after the merger may correlate strongly with the violation of linear momentum conservation, indicating that we cannot reject the possibility that the excitation of the one-armed mode has a numerical origin.

astro-ph.HE↗

arXiv2025

Black hole-accretion disk collision in general relativity: Axisymmetric simulations

Motivated by recent discoveries of X-ray quasi-periodic eruptions, we revisit the collision of a black hole and an accretion disk. Assuming that they are orbiting a supermassive black hole in orthogonal orbits, we perform a general relativistic simulation of the collision, varying the relative velocity $V_0$ from $0.032c$ to $0.2c$ (where $c$ is the speed of light) with a variety of disk thickness and a realistic local density profile for the disk. Our findings indicate that the mass of the outflow matter from the disk, $m_{\rm ej}$, is slightly less than the expected value. Meanwhile, the typical energy associated with this outflow $E_{\rm ej}$ is $\sim m_{\rm ej}V_0^2$. Thus, the predicted peak luminosity from disk flares is approximately equal to the Eddington luminosity of the black hole, whereas the peak time and duration of the flares, which are $\propto m_{\rm ej}^{1/2}$, are shorter than that previously believed. We also demonstrate that the property of the outflow matter induced by the incoming and outgoing stages of the black hole collision is appreciably different. We find that a high mass accretion rate onto the black hole from the disk persists for a timescale of $\sim 10^6$ Schwarzschild time of the black hole after the collision for $V_0/c \lesssim 0.1$, making this long-term accretion onto the black hole the dominant emission process for black hole-disk collision events. Implications of these results are discussed.

astro-ph.HE↗

arXiv2025

Viscous accretion and ejection from tori around black holes in general relativity

We systematically perform long-term (millions of Schwarzschild time) axisymmetric viscous hydrodynamics simulations for tori around black holes in general relativity supposing the super Eddington accretion flow. The initial condition for the tori is modeled simply by the Fishbone-Moncrief torus with a constant specific angular momentum $j$ but with a wide variety of $j$. We find that for a given density profile, the fraction of the mass infall onto the black hole is approximately proportional to $j^{-1}$, indicating that only a minor fraction of the matter in the torus formed far from the black hole falls into the black hole while the majority is ejected with the typical average velocity of a few percent of the speed of light. We also find that the mass ejection is driven only outside $\approx 2\,r_\mathrm{ISCO}$ where $r_\mathrm{ISCO}$ is the areal radius of the innermost stable circular orbit around black holes, which depends strongly on the black hole spin. We derive an approximate fitting formula for the spin-dependence on the mass infall fraction as $\propto r_\mathrm{ISCO}^{0.7}$, which suggests that the rapid growth of supermassive black holes proceeded primarily by the accretion of the matter with the angular momentum counter-rotating with the black hole spin.

astro-ph.HE↗

arXiv2025

Axisymmetric stability of neutron stars as extreme rotators in massive scalar-tensor theory

Differentially rotating scalarized neutron stars, mimickers of binary merger remnants, can possess an enormous angular momentum larger than what could possibly be sustained in a neutron star in general relativity by about one order of magnitude. A natural question to ask is whether these solutions are stable and thus can realize in a binary coalescence. With this motivation in mind, we examine the criterion of dynamical stability against axisymmetric perturbations for these ultra-rotators by numerically tracking their nonlinear evolution in an axisymmetric setup. We demonstrate that the turning-point criterion still serves as a sufficient condition for asymmetric (in)stability. Our findings open an interesting question of whether the merger of two scalarized neutron stars can produce (possibly short-lived) ultra-highly rotating merger remnants.

gr-qc↗

arXiv2025

SACRA-2D: New axisymmetric general relativistic hydrodynamics code with fixed mesh refinement

We present \texttt{SACRA-2D}, a new MPI and OpenMP parallelized, fully relativistic hydrodynamics (GRHD) code in dynamical spacetime under axial symmetry with the cartoon method using the finite-volume shock-capturing schemes for hydrodynamics. Specifically, we implemented the state-of-the-art HLLC Riemann solver and found better accuracy than the standard Total Variation Diminishing Lax-Friedrich Riemann solver. The spacetime evolves under the Baumgarte-Shapiro-Shibata-Nakamura formalism with Z4c constraint propagation. We demonstrate the accuracy of the code with some benchmark tests and excellent agreement with other codes in the literature. A wide variety of test simulations, including the head-on collision of black holes, the migration and collapse of neutron stars, and the collapse of a rotating supermassive star to a massive black hole and a disk, is also performed to show the robustness of our new code.

astro-ph.HE↗

arXiv2025

Self-consistent scenario for jet and stellar explosion in collapsar: General relativistic magnetohydrodynamics simulation with dynamo

A resistive magnetohydrodynamics simulation with a dynamo term is performed for modeling the collapsar in full general relativity. As an initial condition, a spinning black hole and infalling stellar matter are modeled based on a stellar evolution result, superimposing a weak toroidal magnetic field. After the growth of a massive torus around the black hole, the magnetic field is amplified in it, developing poloidal fields via dynamo. In an early stage of the torus growth, magnetic fluxes that fall to the vicinity of the central black hole are swallowed by the black hole and global poloidal magnetic fields that can be the source of the Blandford-Znajek mechanism are not developed. However, in a later stage in which the ram pressure of the infalling matter becomes weak, the magnetic field amplified by the black hole spin via the winding becomes large enough to expel the infalling matter by the magnetic pressure, and subsequently, a global poloidal magnetic field that penetrates the black hole is established, launching a jet along the spin axis by the Blandford-Znajek mechanism with the luminosity suitable for explaining typical long gamma-ray bursts. Together with the jet launch, the effectively viscous effect in the inner region of the torus and the magnetocentrifugal effect drive the stellar explosion with the explosion energy comparable to typical or powerful supernovae. We also find large amounts of synthesized $^{56}$Ni and Zn associated with the stellar explosion. In the presence of jet launching, $r$-process elements are weakly synthesized. The numerical results of the explosion energy, ejecta mass, and $^{56}$Ni mass are in a good agreement with those for observed broad-lined type Ic supernovae. Our result illustrates a self-consistent scenario for the gamma-ray-burst-associated broad-lined type Ic supernovae.

astro-ph.HE↗

arXiv2024

Accessing universal relations of binary neutron star waveforms in massive scalar-tensor theory

We investigate how the quasi-universal relations connecting tidal deformability with gravitational waveform characteristics and/or properties of individual neutron stars that were proposed in the literature within general relativity would be influenced in the massive Damour-Esposito-Farese-type scalar-tensor gravity. For this purpose, we systematically perform numerical relativity simulations of ~120 binary neutron star mergers with varying scalar coupling constants. Although only three neutron-star equations of state are adopted, a clear breach of universality can be observed in the data sets. In addition to presenting difficulties in constructing quasi-universal relations in alternative gravity theories, we also briefly compare the impacts of non-general-relativity physics on the waveform features and those due to the first order or cross-over quantum chromodynamical phase transition.

astro-ph.HE↗

arXiv2024

Binary neutron star mergers in massive scalar-tensor theory: Properties of post-merger remnants

We investigate the properties of post-merger remnants of binary neutron star mergers in the framework of Damour-Esposito-Farese-type scalar-tensor theory of gravity with a massive scalar field by numerical relativity simulation. It is found that the threshold mass for prompt collapse is raised in the presence of the excited scalar field. Our simulation results also suggest the existence of long-lived $ϕ-$mode in hypermassive neutron stars due to the presence of the massive scalar field which enhances the quasi-radial oscillation in the remnant. We investigate the descalarization condition in hypermassive neutron stars and discover a distinctive signature in post-merger gravitational waves.

gr-qc↗

arXiv2024

Bridging relativistic jets from black hole scales to long-term electromagnetic radiation distances: A moving-mesh general relativistic hydrodynamics code with the HLLC Riemann solver

Relativistic jets accompany the collapse of massive stars, the merger of compact objects, or the accretion of gas in active galactic nuclei. They carry information about the central engine and generate electromagnetic radiation. No self-consistent simulations have been able to follow these jets from their birth at the black hole scale to the Newtonian dissipation phase, making the inference of central engine property through astronomical observations undetermined. We present the general relativistic moving-mesh framework to achieve the continuity of jet simulations throughout space and time. We implement the general relativistic extension for the moving-mesh relativistic hydrodynamic code, JET, and develop a tetrad formulation to utilize the Harten-Lax-van Leer Contact (HLLC) Riemann solver in the general relativistic moving-mesh code. The new framework is able to trace the radial movement of relativistic jets from central regions where strong gravity holds all the way to distances of jet dissipation.

astro-ph.HE↗

arXiv2023

General-relativistic radiation transport scheme in Gmunu II: Implementation of novel microphysical library for neutrino radiation -- Weakhub

We introduce \texttt{Weakhub}, a novel neutrino microphysics library that provides opacities and kernels beyond conventional interactions used in the literature. This library includes neutrino-matter, neutrino-photon, and neutrino-neutrino interactions, along with corresponding weak and strong corrections. A full kinematics approach is adopted for the calculations of $β$-processes, incorporating various weak corrections and medium modifications due to the nuclear equation of state. Calculations of plasma processes, electron neutrino-antineutrino annihilation, and nuclear de-excitation are included. We also present the detailed derivations of weak interactions and the coupling of them to the two-moment based general-relativistic multi-group radiation transport in the \texttt{G}eneral-relativistic \texttt{mu}ltigrid \texttt{nu}merical (\texttt{Gmunu}) code. We compare the neutrino opacity spectra for all interactions and estimate their contributions at hydrodynamical points in core-collapse supernova and binary neutron star postmerger remnant, and predict the effects of improved opacities in comparison to conventional ones for a binary neutron star postmerger at a specific hydrodynamical point. We test the implementation of the conventional set of interactions by comparing it to an open-source neutrino library \texttt{NuLib} in a core-collapse supernova simulation. We demonstrate good agreement with discrepancies of less than $\sim 10\%$ in luminosity for all neutrino species, while also highlighting the reasons contributing to the differences. To compare the advanced interactions to the conventional set in core-collapse supernova modelling, we perform simulations to analyze their impacts on neutrino signatures, hydrodynamical behaviors, and shock dynamics, showing significant deviations.

astro-ph.HE↗

arXiv2023

Outflow energy and black-hole spin evolution in collapsar scenarios

We explore the collapsar scenario for long gamma-ray bursts by performing axisymmetric neutrino-radiation magnetohydrodynamics simulations in full general relativity for the first time. In this paper, we pay particular attention to the outflow energy and the evolution of the black-hole spin. We show that for a strong magnetic field with an aligned field configuration initially given, a jet is launched by magnetohydrodynamical effects before the formation of a disk and a torus, and after the jet launch, the matter accretion onto the black hole is halted by the strong magnetic pressure, leading to the spin-down of the black hole due to the Blandford-Znajek mechanism. The spin-down timescale depends strongly on the magnetic-field strength initially given because the magnetic-field strength on the black-hole horizon, which is determined by the mass infall rate at the jet launch, depends strongly on the initial condition, although the total jet-outflow energy appears to be huge $>10^{53}$ erg depending only weakly on the initial field strength and configuration. For the models in which the magnetic-field configuration is not suitable for quick jet launch, a torus is formed and after a long-term magnetic-field amplification, a jet can be launched. For this case, the matter accretion onto the black hole continues even after the jet launch and black-hole spin-down is not found. We also find that the jet launch is often accompanied with the powerful explosion of the entire star with the explosion energy of order $10^{52}$ erg by magnetohydrodynamical effects. We discuss an issue of the overproduced energy for the early-jet-launch models.

astro-ph.HE↗

arXiv2023

Supernova-like explosion of massive rotating stars from disks surrounding a black hole

We perform a new general-relativistic viscous-radiation hydrodynamics simulation for supernova-like explosion associated with stellar core collapse of rotating massive stars to a system of a black hole and a massive torus paying particular attention to large-mass progenitor stars with the zero-age main-sequence mass of $M_\mathrm{ZAMS}=$20, 35, and 45$M_\odot$ of Ref.~\cite{Aguilera-Dena2020oct}. Assuming that a black hole is formed in a short timescale after the onset of the stellar collapse, the new simulations are started from initial data of a spinning black hole and infalling matter that self-consistently satisfy the constraint equations of general relativity. It is found that with a reasonable size of the viscous parameter, the supernova-like explosion is driven by the viscous heating effect in the torus around the black hole irrespective of the progenitor mass. The typical explosion energy and ejecta mass for the large-mass cases ($M_\mathrm{ZAMS}=35$ and $45M_\odot$) are $\sim 10^{52}$ erg and $\sim 5M_\odot$, respectively, with $^{56}$Ni mass larger than $0.15M_\odot$. These are consistent with the observational data of stripped-envelope and high-energy supernovae such as broad-lined type Ic supernovae. This indicates that rotating stellar collapses of massive stars to a black hole surrounded by a massive torus can be a central engine for high-energy supernovae. By artificially varying the angular velocity of the initial data, we explore the dependence of the explosion energy and ejecta mass on the initial angular momentum and find that the large explosion energy $\sim 10^{52}$ erg and large $^{56}$Ni mass $\geq 0.15M_\odot$ are possible only when a large-mass compact torus with mass $\gtrsim 1M_\odot$ is formed.

astro-ph.HE↗

arXiv2023

Binary neutron star mergers in massive scalar-tensor theory: Quasi-equilibrium states and dynamical enhancement of the scalarization

We study quasi-equilibrium sequences of binary neutron stars in the framework of Damour-Esposito-Farese-type scalar-tensor theory of gravity with a massive scalar field, paying particular attention to the case where neutron stars are already spontaneously scalarized at distant orbits, i.e., in the high coupling constant case. Although scalar effects are largely quenched when the separation $a$ is $\gtrsim 3$--$6$ times of the Compton length-scale that is defined by the scalar mass, we show that the interaction between the scalar fields of the two neutron stars generates a scalar cloud surrounding the binary at the price of orbital energy when $a \lesssim 3$--$6$ times of the Compton length-scale. This enables us to constrain the scalar mass $m_ϕ$ from gravitational-wave observations of binary neutron star mergers by inspecting the dephasing due to such phenomenon. In particular, the event GW170817 is suggestive of a constraint of $m_ϕ\gtrsim 10^{-11}$ eV and the coupling strength should be mild if the neutron stars in this system were spontaneously scalarized.

gr-qc↗